Think of your phone as a safe, and your fingerprint as the key.

With a warrant the police cannot force you to divulge the location of the key to your safe, but they can absolutely open your safe if they find it. (Yes they can pick the safe or cut it open but that's irrelevant to this analogy) If your safe uses a code to unlock they cannot force you to give them the code. (Again yes they'll get into the safe eventually, somehow, but irrelevant).

It then follows that with a warrant the police cannot force you to divulge if you use facial recognition or which finger you use, but they can just try things until it works. And again they cannot force you to give them a pin or passcode to your phone.

Key concept here is don't use fingerprints or facial recognition to secure your phone.