The idea of a passkey is that it is a security certificate that permanently bound to the software/hardware and can't be exfiltrated, in the same fashion you'd make one SSH private key per device connecting to a server, never leaving the computer it was generated from. Or how you'd keep your primary PGP keys in a safe location and deploy a unique subkey per device to use it. That way you can revoke an individual subkey if compromised, without revoking the entire chain.
You don't backup your Passkeys, you associate multiple passkeys per account (ie: ProtonPass, Bitwarden, Yubikeys) as a contingency.