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.
Sadly, in this world you accomplish nothing for being nice and considerate. If you want to leave an impact (anything - a new invention, a new product, a new idea, anything with impact to contemporary culture) you have to bully yourself to the top, including stealing ideas and screwing people over, as well as to exploit people. All "great" people who accomplished something did that: Gates (Microsoft), Jobs (Apple), Musk (Tesla, Twitter), Bezos (Amazon), Thiel (PayPal, Palantir), Zuckerberg (Meta), Huffman (Reddit), as well as many politicans. It's a personality treat.
Here is a video that explains the issue, albeit it focuses on designers:
Jesus only in Bavaria. But otherwise you're right.
An anecdote: Back when I was studying we had the opportunity to gain a temporary full version of a specialised software. All there was to do was to proof that one is a legit student. We had to submit our proof of enrollment to the software's manufacturer. The only way to do so was to submit it via fax! It coldn't be done with email, as they told us on the telephone hotline. The software was a German product. We already have been overtaken by the rest of the world (regarding the IT sector).
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.