Why would they start building a full Artemis mockup when the generic Starship upper stage hasn't been finalised yet? That makes absolutely zero sense, at the very least they need to achieve orbit and confirm their design works before building a moon lander. You have to walk before you can run.

There's lots of publicly accessible information on the subject, SpaceX in particularly has also paid Russia for access to Soviet rocketry, and most importantly they hire personnel from existing rocket programs who are experienced in their field.