I think the intended workflow is you login with your phone and that device is now the authority that allows other devices to issue their own passkeys.

In my opinion it's a bad plan, because it elevates certain devices to privileged status, if you lose your phone you are hosed.

Passkeys should be allowed to be synced between devices and stored on password managers in the cloud. I am making my own password manager for my personal use, but have not delved into passkeys.