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.
But is it possible in the implementation of Android/iOS?
Backups are a thing. With SSH keys I have different key for every device too, but as they are stored in an accessable file (as all computer data should be) they are backed up with the rest of the system.