Because pressing it means looking at a screen instead of at what's in front of you. The gesture is the whole interface: phone against your chest, session starts. Lift it away, session ends.
Nothing. The screen is black and empty on purpose, facing your shirt. What you get is a heartbeat: one beat the instant you make contact, then one for every photo taken.
Three pitch-black frames in a row and the session ends itself. Same when storage runs low.
Straight into your photo library at the end of the session. Nothing is uploaded, and there is no gallery inside the app to go back through.
There's a Capture intent for the Action Button, Shortcuts, and Siri. It opens the app ready to go. Proximity is still what actually begins the session.
Yes, open source and free to use.