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.
My understanding of Apple Keychain is that every credential is useable from every device, and can be backed up and restored to a new device. Most importantly Apple doesn’t have access, although we have to trust them on that