Restarting an Android phone (by holding down the power button) usually requires a user to enter a numeric passcode, giving them access to the phone. Then they can add additional fingerprints to provide them with access in the future.
My old phone used a fingerprint reader. When I was on chemotherapy, the skin on my fingers developed lots of little creases, meaning that I struggled to get it to recognise me. I got into the phone by restarting it, then deleted the virtually useless fingerprint and started again but providing the phone with fingerprints from four different fingers, so that I stood a better chance of being able to get in thereafter.
My current phone uses facial recognition but, whenever I try to open it, it also offers me a keypad to enter my passcode as an alternative way of getting in. I've found that to be particularly useful in these days of wearing face masks a lot of the time!