I don’t know how they do it but in general the big thing you need to think about is how to handle reactivity.

There is no reason you cannot implement MVVC/MVC or whatever else on JS for reactivity, it’s just that react et al just abstracts that away for you.

You are effectively choosing some method of implementing and abstracting the observer pattern and handling cleanup since you can easily get into a situation where you have dangling references to objects that are never cleaned up (memory leaks) or pretty nasty GC hangs which can make the UI feel bad to use.