(Sorry I was putting the baby to bed).
OK - so the first thing is that screen time is a big problem for ADHD children. It holds their attention extremely well because of the way most apps and devices are designed and tends to cause them to go into "hyperfocus" - where they almost blot everything else out around them, often for hours if unchecked, and in addition tend to act like an addict when they aren't on it - constantly seeking for more time or looking towards the next time they are allowed.
So it's quite important to limit screen time and be very consistent/strict about it, as well as modelling and encouraging positive ways to use the screen type devices rather than letting him use it as a way to zone out/fill up his boredom (you might have already noticed this distinction). It's very important for children to have chance to develop their imagination/creativity and screen time doesn't do this when they are using it in that way.
A couple of things to note: Children with ADHD really struggle with transitions, that is changing from one activity or state to another, so stopping something to start a new task. You can help with this by seeing the process as a multi stage one. So for him for example if he has to stop playing the switch because it's dinner time, that's several things - 1. Recognising his current activity is ending. 2. detaching from current activity, 3. identifying new activity and transferring focus without getting sidetracked, 4. engaging with new activity.
When this is NOT managed, this looks like
1: ignoring you when you ask him to come off, whining that it's not fair, saying he just needs to finish this level/match/race.
2: Taking ages to actually stop. Claiming to be searching for a save point but "Oops" just missing one. Having a giant tantrum if you take it off him.
3: "Getting lost" on the way to the table. Spending ages washing his hands because he's playing with the water. Getting sidetracked by a magazine.
4: Fidgeting in seat, playing with food for ages before starting to eat, needing to get a drink, a napkin, a different fork, ketchup before he can start.
You can help by giving pre warnings that something is coming to an end. With screen time because it's so compelling it helps to get to know the games he likes and suggest a natural end-point, rather than using a time limit in minutes. When he's very engrossed in an activity he might literally not hear you so it helps to make sure you have his attention first - say his name, come down to his level, look him in the eye, put your hand on his shoulder, etc.
Praise when he does something straight away, rather than getting cross when he doesn't do something straight away. Allow a transition time between tasks and build this in so you don't get frustrated. Also - give him one instruction at a time. "Put your pyjamas on and clothes in the basket" is quite a lot - a neurotypical 7 year old might be able to handle that, but he would probably benefit from breaking it down into "Take off your clothes" "Put them in the basket" "Choose some pyjamas" "Put them on". Again praise if he does do it. If it helps, get him to pretend he's a robot and he has to take an input of one instruction at once. He'll probably like that game. You can also joke that he's malfunctioning if he does stray off task. And make sure to go down to his level and make eye contact for the first instruction, and bear in mind if it's a transition he'll need coaching away from the first activity first - if he's just arrived in his room after tooth brushing or whatever, you might be able to start with taking clothes off. It also helps if all tasks come in the same order every time, rather than having things change. Broken record technique until something happens - simply repeat instruction in boring, calm voice, refuse to engage with anything else.
For things like the baby that he repeatedly gets wrong - coaching/helicoptering. Essentially don't leave them unattended. When he is allowed to play with him supervise and again one step instructions of a suitable game - pick something that it doesn't matter if he gets too excited and/or where it is difficult to get too excited. Or get him to do his silly/excited things at a distance from the baby. Baby can watch big brother do a dance or a trick (babies love that kind of thing). Or side by side activities - get him to draw a line picture that the baby is allowed to "colour in", build a tower for the baby to knock over, and so on.
Pick your 5 biggest battles - work out what the opposite behaviour is that you want e.g.
Not listening = for task to be completed straight away without moaning/repeating
Manhandling the baby = to play in appropriate manner with baby
Then you can notice when it's happening right and praise. It also means you can look at the issue and see whether your expectations (the wanted behaviour) is reasonable and/or what you can do to make that easier and/or more appealing for him - so breaking tasks down and making sure you have his focus to begin with makes that one easy, and for baby playing perhaps he doesn't know how to play appropriately, what does that look like, can you show him etc.
Is all of this exhausting? YES, it definitely is, for him as well as you. So another thing is to set yourself low targets and easy wins, don't try to fight too many battles all at once. If you have a partner, tag team, because with a toddler as well it will be a lot. Push for diagnosis as therapy and medication might be appropriate. Unashamedly use after school activities (if he can cope with them/they can cope with him) as childcare to give yourself a break. Plenty of exercise and fresh air for him (it helps) and enough sleep and/or an enforced bedtime after which he does not get attention (he can read quietly, etc) and experiment with allowed amounts of screen time, but most ADHD families find very little is best. (This is my big failing ATM).
Hope this is helpful - good luck.