I know this is hard but you need to think of her behaviour as communication. She is struggling with the world around her and expressing her distress outwardly.
It sounds like a neurodevelopmental assessment would be really helpful right now. It would involve various professionals observing her and meeting you. They would note any aspects of development where she appears to be struggling, things that cause her stress and ways in which she may be struggling to express herself. They would then feed back to a panel who would decide whether a diagnosis would be appropriate and make recommendations for how she could be supported/treated.
That's not going to happen at the moment so I would suggest that you read around ADHD and autism for a start. Try some of the strategies recommended for children with these difficulties and take note of what helps.
I have two DDs with autism and my advice would be to reinstate a routine. Make it one with as few demands as possible and that you can generally avoid disrupting.
If you need to change the routine, give her lots of notice and let her know exactly what will be happening instead.
It's very easy to avoid busy environments at the moment but she may need breaks from social interaction, even in your home.
Give her a quite place, like a den, with some fiddle toys where she can withdraw when she's feeling overwhelmed.
Try to avoid strong smells, loud noise, unfamiliar food, uncomfortable clothing and other intense sensory input.
When she loses it. don't tyr to communicate too much with her. Don't punish her for these meltowns but, hours later, support her kindly and positively to apologise and make amends. Assume she feels bad about what she did.