A passkey is just web authentication credential registered to have certain pre-existing capabilities so that you can have a particular user experience.
Passkeys don't rely on this mechanism (although being able to authenticate your desktop using your cellphone is a very valuable user experience!)
This upcoming mechanism is also meant to be used between platforms (or at least, a platform and an extremely robust authenticator). It is not meant to be used by a website or native application which want to just rely upon/consume credentials. If the browser/platform supports this mechanism, it will be presented to the user as a choice of how to authenticate.
Passkeys don't rely on this mechanism (although being able to authenticate your desktop using your cellphone is a very valuable user experience!)
This upcoming mechanism is also meant to be used between platforms (or at least, a platform and an extremely robust authenticator). It is not meant to be used by a website or native application which want to just rely upon/consume credentials. If the browser/platform supports this mechanism, it will be presented to the user as a choice of how to authenticate.