If you find yourself in a situation where you might be detained and thereby unable to control interactions with your device, on iPhone you can disable TouchID and FaceID by holding one of the volume buttons and the sleep/wake button simultaneously for a few seconds. This will require you to enter your passcode the next time you want to unlock iPhone. Anyone have similar instructions for Android?
Better advice is to read the situation. Cops can make, and have made, people's lives very difficult, whether through legal harassment, false charges or bodily harm, and it is your word against a cop's when you go before a judge.