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.
The irony is that it's an open standard. There are FOSS implementations you can self-host. Server side, client side, soft token, hard token. Everything.
https://github.com/herrjemand/awesome-webauthn
People on this thread are just really ignorant, even self-proclaimed security experts.