For iPhone (and iPad) owners:

How to Temporarily Disable Face ID or Touch ID, and ...

Absolutely essential anytime your phone might leave your possession.

For folks in the US: if your phone is locked with Face ID or Touch ID, the cops can compel you to unlock it. If it's locked with a passcode, they can't force you.

(Obligatory "I am not a laywer, check local laws" here.)

And remember: Never talk to the cops. Never consent to a search. Stay silent and ask for a lawyer.