To add a piece to this puzzle: Pilots who build their own airplane from scratch often apply for, and receive a mechanic’s certificate, specific to their own aircraft. That gives them the legal authority to do ANY mechanical work on their own airplane. Of course, it would be silly to require that pilot/mechanic to become a full-time mechanic with an established shop, just so he can service his own airplane. That is one of the reasons pilots build their own airplane, to be able to do the mechanical inspections and work themselves.