It sits right after the flags and counter in attested credential data, before the credential ID length. Hardware keys report the AAGUID of their model; password managers and platform authenticators publish theirs too (community-maintained lists map them to names such as iCloud Keychain or Google Password Manager). When attestation is none, many platforms return all zeros.
Without verified attestation the AAGUID is just a claim made by the client and can be spoofed, so use it for display ("signed in with your YubiKey 5") and analytics, not for access decisions; for policy enforcement, verify attestation against the FIDO Metadata Service. The AAGUID lookup maps values to authenticator models.