There are two types of passkey. Syncable and device-bound. (see fidoalliance.org/passkeys/). Theoretically, the device-bound passkeys never leave the device and users don’t have any access to it except to use it for authentication. The syncable type will first and foremost be synced by the platforms themselves (Google, Microsoft, and Apple), but eventually the 3rd-party password managers will be allowed to be sync providers, but possibly only on newly-released OSes.

As far as I know, the passkey implementations currently on Android and Windows are device-bound; they are not synced to the cloud.