I’m pretty sure this difference isn’t real. On both, the UI is supposed to be for the UI and anything that takes longer is supposed to happen on a different thread. Even Windows Phone had that. However, in practice developers don’t always do it and this isn’t as great as it sounds. If you’re scrolling or something and scroll faster than the background threads, it will stutter. If the app has a resource leak, it will stutter. If the graphics are too complicated, it will stutter.

RAM requirements depend on what you’re doing. I had a Pixel 4 and it always ran great. I had to get rid of it because it was physically falling apart and Google stopped releasing security updates for it.