There's an iOS feature called Guided Access[1], which allows you to hand over your phone in a limited-access state (locked to the current app, optionally with areas of the screen disabled for input - or whole input classes like touch/keyboard/motion/volume control disabled). You can also apply a time limit, after which time the phone requires you to authenticate (you can even prohibit Face ID to exit guided access, requiring a PIN instead).
Once enabled and in an app, you just triple-click the side button to start
It's not designed for fully-untrusted users, of course - my understanding is that no in-memory key storage is invalidated. It's useful for handing a phone to friends and relatives to browse some photos with limited access to the rest of your files, and without notifications appearing