Disable face/fingerprint unlock (biometrics)
You have stronger legal standing with a passcode than with biometric unlock. Cops can try to force you to unlock your device with biometrics.
How to disable face/fingerprint unlock
On iPhone: Settings > Face ID & Passcode > Disable "Use Face ID for iPhone Unlock" (you can leave the rest enabled)
On Android: Settings > Lock Screen (or Security) > Biometrics and Security > Disable both "Face Recognition" and "Fingerprint Unlock" for unlocking your phone
Caveat: Face and fingerprint unlock let you use stronger passwords without typing them all the time. If you have to choose, it's better to have a longer passcode than to turn off face/fingerprint unlock.
iPhone locking tip: If you leave face/fingerprint unlock on, you can force your passcode to be used for the next unlock by pressing the side button 5 times.
iPhone camera tip: You can take photos without unlocking your phone by swiping left on your home screen. (In case you get arrested while filming.)