Opened the camera, phone froze.
In case it happens to you, you aren’t the only one.
Step 1 : Quickly press and release the Volume Up button.
Step 2 : Quickly press and release the Volume Down button. (Do not press and hold, simply press and let go)
Step 3 : Press and hold the Side button . The Slide to Power off button will appear but continue to hold the Side button until you see the Apple logo on the screen.
Steps above from here, with lots of superfluous text.
My phone restarted itself while I was searching for instructions on how to restart it. Apple thinks of everything.