Google Authenticator is a standards-based implementation of TOTP. When enrolling the factor, a “shared secret” is generated by Okta and stored in Google Authenticator as part of the enrollment process (e.g. scanning QR code or manually creating a new entry). Okta asks you to complete a TOTP challenge as part of the enrollment process to make sure you the end-user has successfully configured their authenticator.
You can only obtain the “shared secret” during factor enrollment.