When iphone duo opens and closes, how does the camera automatically decide which camera to use, and what new possibilities does this create for users?
Duo Camera
Have you watched https://developer.apple.com/videos/play/tech-talks/111465/ ?
This should answer these questions well. In short — the built in (virtual) front camera automatically switches on your behalf. Or you can update your app to use the inner and outer front cameras separately and unlock the full feature set of each camera.