We had originally planned to go all-in on passkeys for ONCE/Campfire, and we built the early authentication system entirely around that. It was not a simple setup! Handling passkeys properly is surprisingly complicated on the backend, but we got it done. Unfortunately, the user experience kinda sucked, so we ended up ripping it all out...
Yes you can use a passkey set up on any given service to authenticate to a service that supports passkeys. You’d need import/export to move a given passkey from bitwarden to Windows.