OK I am back with resources. Sorry this is really long. I wouldn't expect anybody to be able to absorb all of this info at once nor will all the resources be relevant but they are all things I've found helpful at various times and I would say maybe pick one of these ideas/concepts/suggestions to explore.
First of all observe. Observe observe observe. You're looking for patterns of behaviour.
There is a framework called Antecedent, Behaviour, Consequence which is very useful - you can do something called an ABC plan. To do an ABC plan you want to work out what behaviour it is you're trying to change and that means noting down the unwanted behaviour but also if possible (and it might not be yet) the positive replacement behaviour you'd like to see instead. A positive replacement behaviour is something like - if child usually hits other children, using words to communicate. If they are using words a lot then you might need to narrow it down to specific situations they tend to hit.
Then you can observe your child in various environments and if you see either the unwanted behaviour, or any attempt at the positive replacement behaviour, and note down the antecedent (ie, what happened immediately before or what seemed to prompt this behaviour) and the consequence (ie, what happened immediately afterwards as a result of the child's behaviour, this could be positive or negative). Often you notice that for example the attempts at the positive replacement behaviour get overlooked or are not effective, or the child gets no attention or praise for this, but the unwanted behaviour gets immediate attention, even if it is being told off. This is unbalanced because young children's brains are wired to find attention very reinforcing. It's not that they are seeking out being told off because they "want" this, it's that their brain has then sort of marked that behaviour as important and valuable because it achieves something.
This video course is really great in terms of how to do these observations and identify positive opposite behaviours and a lot of really excellent tips of how to more effectively reward the behaviour you want to see, including my favourite tip which is about breaking things down into much smaller steps and getting there at the child's pace. https://www.youtube.com/playlist?list=PLh9mgdi4rNeyEGNxBvNdOVlianDYgWuc9
But also if you find that there is an antecedent which frequently triggers a certain behaviour in the child, that is likely indicative of a situation they lack the skill to manage effectively. Obviously as a parent you won't necessarily have the skills or knowledge to identify which skills are missing, but sometimes it's clear because you can see that the way they are asking for something is not effective, or that they are misinterpreting a situation with a similar pattern over time, and so this gives you information that you might be able to use to practice that skill outside of that situation. At 5 some really good ways to practice skills related to basic things like manners/polite tone, turn taking, flexibility (ie, letting other people decide sometimes), emotion recognition etc, are role play, either with toys or by taking different roles in games like Shops, Restaurant etc. You can also build phrases to practice into simple board/card games. Or sometimes you can set up opportunities to practice in an easier situation. For example, if your child struggles with supermarkets, don't start with expecting them to tolerate an entire weekly shop on a busy Saturday, in between other errands or activities. Go in at a quieter time, or to a smaller shop, with a very short list and take it slowly, one step at a time. If you can see that your child is struggling socially but don't know exactly what with, you can also buy various books with guidance on how to support development of social skills or cognitive skills like waiting, flexibility, etc. The "Out of Sync Child" series tends to be good but I'm sure there are other useful books. And I've often found that I get a good starting point if I write into a chatbot describing common issues or situations my child struggles with along with their age and asking what underlying skill(s) could be involved, and what the developmental path is for those skills (ie, what is even earlier that those skills build on). This will give me language and keywords to search and I can then find websites, articles, or sometimes books or other sources of advice and suggestions about how to support and develop that area for my child. Oh, here I also like Linda K Murphy's The Co-Regulation Handbook.
Of course, it also goes without saying that if you are worried that your child is not developing appropriate skills at the same rate as their peers, always get advice from the GP or Health Visitor or some areas have Family Hubs. If they tell you to wait and see, then ask for a time scale on this and what specifically to look out for. So for example if they say this is very common and children usually grow out of it, ask at what stage you should come back if the behaviour or difficulty persists. Or if they say this is likely temporary stress relating to e.g. starting a new school, say how long should you give it to settle down. Then make a note somewhere perhaps on a calendar, and follow up if you do not see improvement, or if you notice the behaviour or difficulty is getting worse. If you can afford private speech therapy or OT, IME that can sometimes be more useful than paying for private assessment for ND conditions. Behavioural therapy can also be appropriate but ideally it should be training the adult to use skills to support the child's behaviour so you need to be open to this. Parent coaching may also be similar although I think the term is totally unregulated. It might be cheaper for that reason though.
In some situations you find that the ABC framework is too simplistic because the antecedent is more of a "last straw" situation and actually it appears quite random or unpredictable. This can be indicative that what you're seeing is more general stress behaviour or dysregulation, and not that there are specific situations your child struggles to navigate.
There are three parts to helping children with stress behaviour/dysregulation IME. Identifying and reducing sources of stress/dysregulation in their environment, where possible. Identifying and increasing access to things which they find regulating or which help calm them, or e.g. "get out" excess energy or big feelings in a healthy way. And lastly (or maybe first?), learning to recognise where they are on a sort of "scale" of regulation in order to help guide them to use appropriate tools at the right time. Eventually the aim is that they can learn to recognise this themselves but I think personally it is unrealistic to expect children to go from being dysregulated a lot of the time to being able to recognise and manage their own state of regulation very quickly. They are likely to need the support of trusted adults for a long time before they can internalise these skills. IME adults (especially in schools, which I understand because there probably aren't loads of spare adults just hanging around ready to help children regulate, although that is not always exactly what is needed) want this to happen very quickly and assume that when it is not happening quickly enough, this approach is not working and then they want to go back to the control/punitive approach because they feel it will get faster results. However, this can often be counterproductive, because feeling controlled in itself can be dysregulating especially if the expectations are too high for the child to reliably meet.
Children will vary with what they find stressful or dysregulating, but some common themes are unpredictability, uncomfortable sensory input esp overwhelm due to multiple conflicting inputs, overwhelm because instructions or expectations are coming in too fast without time to process, proximity to unknown people (esp in larger groups) or people they are wary of, difficult emotions, cognitive load, fear-based discipline, pressure to perform (sometimes praise/reward can feel like pressure), feeling controlled/constrained (threat to autonomy), physical aspects e.g. pain/muscle ache, hunger, thirst, tiredness, toilet need (and some children are slower to develop the sense of interoception, which is their internal awareness of these kinds of body signs).
Whereas regulating input again will vary based on the child and also often based on the exact situation, but some things to try could be alone time/quiet, privacy and space, slowing down and giving them more time to process things in general, opportunity for movement esp involving impact (running, jumping, hitting things), "heavy work", feeling genuinely accepted and competent (e.g. having a particular role within the classroom or home, being praised/rewarded), tasks and behaviour expectations matching where they are at so they can be successful (which may also include working on skills to get them to where they can meet expectations), proximity to other people or animals who are calm and regulated (co-regulation), knowing what to expect/what is coming next, order and familiarity, deep pressure (e.g. weighted blanket, tight squeezy hugs) autonomy of choice.
Some useful resources for understanding the subject of regulation are-
Conscious Discipline - love their three brain state model. They have a framework of teaching skills to adults in order to pass them down to children. Largely school based, but a huge wealth of resource material anybody can use. Genuinely accessibly written. Sounds simplistic but it's about patterns over time and it does work.
Mona Delahooke - explains how this links into behaviour and a lot about adult body language and how we can alter it to shift patterns. More of an overview of the topic.
Stuart Shanker's Self-Reg - has a whole model of how different stress "domains" can affect both capacity to handle stress within the other domains and also functioning of other domains. Good for understanding how to avoid overloading children with too many things at once, not hugely practical other than this, but can be a helpful lens to understand some things, especially the idea of fluctuating capacity aka why "But they can do it sometimes, so it means they are choosing not to" can be flawed.
TheOTButterfly - I don't like to recommend social media influencers but she has some really good explanations of sensory processing and has a book coming out called The Sensory Code which I think will be helpful both for children who struggle with sensory input and also about how to use/identify sensory strategies to support regulation.
Linda K Murphy - two books The Declarative Language Handbook and The Co-Regulation Handbook. Her understanding of Co-Regulation is a bit different to other descriptions but it's vv helpful. Declarative Language can be helpful if children struggle with or automatically lash out against feeling controlled. These two and her writing generally is good for understanding control vs autonomy and the concept of giving children processing time.
Big Baffling Behaviours by Robyn Gobbel - great framework for recognising regulation "level" and lots of advice about what to do vs not to do at different "levels" as well as some broad overall advice about how to support regulation generally. This is probably the most practical book I've ever read for how to handle dysregulated behaviour in the moment. Top recommendation.
Books by Dr. Ross Greene - e.g. Raising Human Beings or The Explosive Child, his school-based book Lost and Found or his new one The Kids Who Aren't OK. They all cover the same method with a slightly different framing. This is more of a guide of how to navigate situations where children are being aggressive/defiant/violent because they don't have a better skill to manage the expectation - Dr. Greene says that this ALWAYS means a child is lacking a skill to better manage the situation. However, his method does not explicitly teach skills, or teach you to manage the explosive behaviour. It is more how to manage the situation when your child does not yet have the skill and isn't going to develop it overnight, sometimes by temporarily dropping the demand, sometimes by working out a way to handle the situation for now. I have noticed that people tend to struggle with this book/approach unless they understand this distinction.