I'm sure you could find examples to prove me wrong here so I'm definitely not saying this is a hard line, but I've always found that if app state is too complex to represent in the UI or isn't needed in the UI at all, that's state that belongs on the back end rather than the frontend.

My usual go-to rule is that business logic belongs where the state lives - almost always on the back end for state of any real complexity.

With true web apps like Figma I consider those entirely different use cases. They're really building what amounts to a native app that leverage the web as a distribution platform, it has nothing to do with HTML at all really.