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.
Ummm??? What keychain? Passkeys don’t have physical keys. I think you’d better learn more before cursing Google. BTW Apple supports them as well.
“Keychain” is often used colloquially to refer to a piece of software that holds passwords and other secrets, which can include passkeys depending on the implementation.