A screenshot of a message prompt I got when trying to set up a passkey with one of my online accounts. It doesn't give me the private key to save into the app of my choice, it demands that I use the mobile app on a Google or Apple device to set up and use the passkey.
https://files.mastodon.social/media_attachments/files/113/844/225/137/900/323/original/e20d6ee0cea73d3e.png