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 missed the part where I said I don’t trust Google. You seemed to have ignored everything of substance in my response, namely putting a password on the passkey doesn’t remove passwords and the extension of things like FIDO2 beyond web auth.