Never unlock your phone for cops. Turn off biometrics if you think you'll be encountering police. You can't be forced to enter a password or number to unlock your phone, but can be forced to use fingerprint or face id to unlock.

Once unlocked, they will make a clone of your phone's contents to search through at their leisure. They will access any open social media or other accounts and copy any messages from those also. It doesn't matter if you "have nothing to hide" they can find things to incriminate you or people you know. They can hold that data indefinitely, in case anything you've done becomes a crime later.