Any attempt at trying to figure out how passkeys are going to work in this environment has left me - despite years in the industry - totally baffled.
Very curious about how others who regularly use different operating systems on various hardware platforms have navigated passkeys and whether it's actually worked, or if fall-back to passwords has still been necessary.
Passkeys are not like passwords, where you typically register a single username+password combination for each account you use. Passkeys are more like SSH keys, where you typically register multiple SSH keys for each account you use, one for each device from which you want to access the account.
If you only use a single device or platform, it's possible to register just a single passkey for each online account you use. Even so, most technical-savvy folks will want to register at least two passkeys for each of their important accounts (to at minimum have at least one backup authenticator in case they loose access to the primary authenticator). And for accounts that you regularly access from different devices, if you find it convenient to use the built-in platform authenticator on each of those devices, you would register a separate passkey for the same account on each device.
The FAQ section of this page might help clear up some of the questions you have about passkeys:
It's not as safe as hardware keys, but way more convenient, especially across multiple platforms. I've used it on Mac, Windows, iOS, and Android. Not sure about desktop Linux.
As a bonus, it makes sharing passwords (like Netflix) much easier and safer too.