the user wants to use passkeys (discoverable keys) out of the box - no bitwarden mechanism.
Unreleased microg v3.10.0 will bring along more support for passkey selection.
How does it work..
Credential registrations will be put into the existing fido.db and it will delegate to a transport handler for generation or retrieval of resident keys per registration. Most will use the screenlock transport, putting keys into Androids keystore (or strongbox).
I don’t know if or how pw managers can hook into that for backup & sync. Something needs to backup the fido.db itself too. Comes at a later time?
It’s probably christmas by the time this all lands in an /e/OS version.