When you are arrested, the police will find out who exactly you are. They can do this by simply asking the question or checking your identification. But if you don't want to cooperate, they can also use your fingerprints or face to open your phone and find out who you are.
It is therefore advisable to leave your phone at home and only open it with a PIN code or turn it off when you get the chance. The police cannot open your phone without your explicit permission if it can only be unlocked with a PIN code.