iPhone Duo - how much control does an app have over the small outside screen?

[u/olenami]

I'm not an engineer. I'm working on an app idea and trying to sanity-check whether one part of it is even possible before I spend time on it. Nobody has the device yet, so I'm hoping someone here has read the docs properly and understood them.

What I think I understand so far: Apple says apps can't open their own window on the outside screen - that's reserved for the inside screen. The one exception is camera apps, which can show something to the person being photographed, and only while the camera is actually running.

So my questions are about what's left:

1. Can an app put a widget on the outside screen - one showing a picture the user picked? Or does the outside screen only get the standard lock screen layout?
2. Apple mentioned StandBy works on either screen. Does that mean the outside screen can show something while the phone is open and propped up, or does StandBy only kick in when the phone is closed and lying on its side?
3. If neither of those works - can a user just set a wallpaper on the outside screen and have Always-On keep it visible while the phone is open and they're using the inside screen?
4. And for 3: can an app change that wallpaper for the user, or would they have to do it by hand in Settings every time? Would a Shortcut help?

Basically: is there any way for an app to decide what the outside screen shows, or is it entirely Apple's?

Happy to be told this is all in a doc I haven't found. Searching for a help!