yeah, you can use any app, and they work fine… with, as you might expect some warts due to apps not being designed for that screen size. a common issue is with apps that have ridiculous fixed headers and footers, leaving you a teeny tiny sliver of space for content. there’s an option to make the viewport slightly bigger, since the screen extends down and around the outer cameras, and that helps sometimes, but then obviously the cameras might prevent you from hitting some buttons on that footer. this is not a very common situation though, and maybe almost desirable? I can use the outer screen to do quick phone interactions and put it back in my pocket without being too sucked in. if I want to doom scroll/get otherwise immersed, I’ll open the fold. i like that this needs to be an intentional action.

you can respond to messages and type on the outer screen, yes, using any keyboard as long as it’s Gboard.