Guided Access. Meant for handing your phone to your kid so s/he doesn’t mess anything up. But it’s also useful for keeping that shopping list open at the store when you’re wearing a mask, keeping a recipe on the screen while your hands are messy, etc.
The screen will eventually go off, and you may get the Lock Screen. But if you fiddle with the phone (power button, I think; also try to get to the Home Screen), it will go back to the app you’re locked into.