Usually just an option in addition to a password + MFA. Or they just replace the MFA option and still require a password. I even saw some variants where it replaced the password but still required a MFA code. It’s all over the place. Some providers artificially limit passkeys to certain (usually mobile) platforms.
My password manager supports passkeys just fine, across Windows, macOS, Linux and iOS (and probably Android but I haven’t tried). Surprisingly, iOS integrates with the password manager so it’s usable just like their own solution and it works across the system (not just in the browser).
This seems to be more about finding a standard way to export/import between different password managers/platforms?