Right on the body. More precisely, on its back cover.
This magic is called Back Tap. The idea: tap the back panel twice with your finger, and the phone will do whatever you've assigned to it on its own. Want it to save a screenshot, want it to open the camera. Or turn on the flashlight, or lock the screen. All without a single touch to the front, and sometimes you won't even need to take off the case.
It looks like a light tap, and the iPhone has already understood everything and obediently does what you've assigned. Handy when one hand is busy with a coffee cup and the other urgently needs to capture something — because it's all in the fingers, no tricks involved.
It takes a minute to set up: Settings → Accessibility → Touch → Back Tap. Then choose double or triple tap and assign your action to it.
Our hero here, between repairs, likes to show off tricks like this. Though, in his armoured gloves, he doesn't tap the back panel — he's afraid the camera will open and there he'll be, in all his glory. That's tricky with a matte helmet 😄