You can’t unlock your iPhone with biometrics at first boot, and holding down the two side buttons will make it so your phone immediately disables biometric unlock, and instead requires your passcode for the next unlock.

But none of this has much to do with the biometric auth you do with passkeys, because passkeys are used in places passwords would be used — logging into apps and websites. Which you see only doing when your device is already unlocked and you are actively using it.