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.
I just replaced my iPhone, and the few places I “sign in with Apple” still work in the new phone. Yes, you can back it up and restore to a different device. I assume you can also use it across devices but I haven’t tried that
This is something different to PassKeys. “Sign in with Apple” is Apple telling online service “let him in”, while PassKeys is storing your authentication data on your device.