I don’t know how the procedere would be executed, but I imagine that police could have the phone present during an interrogation and try to nlock it there (possibly by making you to look at the phone to unlock it, if the phone has been set up to unlock this way). Once unlocked, it would be sufficient to have a peek into the camera roll or messages, until the phone locks again. I don’t know about the law, but I can imagine that if a police officer had a look into your phone, even briefly, it may be held against the one who is being interrogated.

Once rebooted, you need to enter your PIN to unlock the phone (and the SIM as well). Before that it is not possible to unlock the phone with biometric credentials (face ID or fingerprint).

As far as I’m aware, police can force you to hand over your biometric credentials (they can hold the phone to your face to unlock it when you have face ID enabled, or can move your finger to the fingerprint sensor). But they can’t force you to reveal the PIN number.