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 don’t think that access to your personal data/email/files being dependent on a battery-powered electronic device is a great idea, to be honest.
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