I work on games that go on many platforms. I've got a secret debug menu that lets me simulate a bunch of screen sizes, then I've got some internal stuff to let me scale just the UI, and control the viewable distance of the in game world (it's all 2d games).

Safe Areas are a PITA because I can't find a reference of every devices safe area.

Our designer is also good at designing things that work across screen sizes.

The mobile world has forced us to get good at this. It's been a few years of struggle though.