Passkeys is the "normal" name for a FIDO2/WebAuthn credential that basically lives within a phone or password manager. It does add a few things. Namely the ability to store many passkeys per device per app/site, the ability to sync those passkeys (e.g. via iCloud or similar), and the ability to use QR codes and Bluetooth to do a local-only authentication on a device which doesn't have the passkey (which is what often requires some proprietary implementation).
[Edit]: An important feature of "Passkeys" is that browsers and operating systems have a special API that allows an app to pre-start a sign in with a known user/email/etc. which if there is a passkey for that user it'll automatically start the FaceID or similar confirmation process. Which Passkeys are checked is controlled by the OS/Password Manager which checks which website is asking and what username it's checking. This is just to make it so it seamlessly logs you in. It does a fall-back to just asking what your user is which is the initial workflow.
This[0] is a good podcast to listen to with Adam Langley from Google about how Chrome supports Passkeys and why they're a good thing. It includes the details of how/where/why there are some proprietary bits needed to implement "Passkeys".
[0]: https://securitycryptographywhatever.buzzsprout.com/1822302/...
FIDO Alliance Press Release https://fidoalliance.org/apple-google-and-microsoft-commit-t...
Chromium Blog on Passkey support (Dec 8, 22) https://blog.chromium.org/2022/12/introducing-passkeys-in-ch...