Passkey is some sort of specific unique key to a device allowing to use a pin on a device instead of the password. But which won't work on another device.
Now I don't know if that key can be stolen or not, or if it's really more secure or not, as people have really unsecure pins.
You have to enter your pin/pattern to re-enable biometrics
Also, I'm not sure which phone you're using, but if it's an Android there should be the option for password, some OEMs don't give that option but they're rare and the standard set by Google is to provide that and also the pins to be very long (I haven't personally checked the limit, but you can make them longer than reasonable)