(31,8)1.A In Medrash Tanchuma; (Nasso 17), the medrash explains that Moshe is given credit for completing the Mishkan despite not having been the one to build it, because he gave of himself more than anyone else, making sure it was built exactly as he had been shown by Hashem. It could be that even though Betzalel knew how to make everything, Moshe had the job of making sure it matched the divine plan. It was for this reason that he needed to know what everything; including the Menorah looked like.