There's already a number of apps in the iOS and android ecosystems that allow this - as far as I can tell this is moving that to default in the OS rather than a third party app.
Yes. If you have an iPhone and a Mac it will just show up as a webcam device automatically.
I have found it quite useful when joining a zoom call with a group of people in a breakout room. Prop up your phone somewhere and it works like a conference room camera.