Yeah, they all download files in a proprietary format so you can only watch using their app, it’s not just a .mp4 that you can use whenever

Actually I doubt it, video in and video out likely needs totally different wiring, so most phones probably couldn’t do it without hardware changes

That would be awesome, although it might make the camera experience worse since you usually use the camera folded. I saw a prototype for a phone a while back that had a regular screen on one side and e-ink on the other, I always thought that was cool.