I've posted a reply to another comment with more details on my question that seemed to have disappeared. So I'm reposting it.
Bitwarden, a popular password manager, now supports passkeys through its browser extension, with plans to extend support to its Android app in the future.
Yes, I've read the article before asking the question.
However I don't find the answer to my question. It will work with the browser extension, not with the android app.
What about the browser extension on Android ? It's not the same thing as the android app.
Especially since Firefox is trying to close the gap between mobile and desktop add-on. It's possible that it could work, even if the extension is not as useful as the app on Android.
Wait, so bitwarden's support is only a mechanism to save other systems passkey? Or is it a mechanism to originate a passkey, as an alternative to Google and friends? My conception of what is going on might be off, as well.. So.. Feel free to enlighten..