How can passkeys be synced when they "never leave your device"?

Reading about passkeys in the iOS change log I get the impression that they're just private keys used to sign requests with, just like physical security keys: > Passkeys never leave your device Sounds reasonable, since the OS would only need an interface for signing — never actual read access to the key. Then I read this: > Passkeys are end‑to‑end encrypted and sync across your Apple devices Uh... Obviously not, since they "never leave your device". How does that work?