Think of passkeys as being the same as a password database. The provider can offer whatever recovery mechanism they want, and sites that use passkeys can continue to offer account recovery methods completely independent of their use of passkeys.
As for what Google does specifically with their implementation, I'm not sure. I personally plan to use KeepassXC's implementation, whenever that comes out, with my own custom database backup strategy.
This is an area with the specs contrast with the vendors.
The WebAuthn specs recommends to register multiple passkeys/credentials per device and assume that once a credential is lost it might not be recoverable.
Apple and other vendors using keychains/wallets are effectively offering the option to delegate the recovery of the passkey to the recovery of the account with them (eg: the iCloud account).
As for what Google does specifically with their implementation, I'm not sure. I personally plan to use KeepassXC's implementation, whenever that comes out, with my own custom database backup strategy.