Passkeys are available for general use in any application through the system frameworks, not just the browser; they simply use WebAuthn under the hood, but it's meant to expand support outside of the browser for more apps in more use cases.

I wrote another comment elsewhere but there are some other issues with using WebAuthn as a primary authentication mechanism right now, especially things like new device enrollment when using platform authenticators (not trivial but not what people are used to), so most people opt to design it as a 2FA mechanism since that normally fits into existing designs easier ("just another auth device" like TOTP or SMS.) So Passkeys will help smooth that a bit for Apple users.

Also we still need ways of exporting keys and software (like 1password) needs to synchronize them, manage them securely. There's still a long ways to go on that front, which probably won't be handled until stuff like this has settled.

If you can use WebAuthn today in the browser and handle new device enrollment and have designed with it in mind as a primary auth mechanism, you're way, way ahead of the curve. This is just Apple's attempt to help kick the can further down the road for their users; you may not need to use Passkeys at all. There's just more to do before we can actually use WebAuthn as the basis to replace passwords in more places.