I’m very new to Bitwarden, but as far as I understand there is no support for passkeys on mobile yet, but it is in development right now. I tried to find where passkeys are supported for Bitwarden in their documentation just now but somehow failed.
At least I found that it says “Passkey support is planned for mobile devices in a future release.” here: Passwordless Future: A Comprehensive Passkeys FAQ | Bitwarden Resources
EDIT: The road map is here: Bitwarden Roadmap