Magearna was grabbed through use of a QR Code in regular Pokemon Sun & Moon, and it's thankfully still the same here in Ultra. One such unedited area of the game is the acquisition of your first mythical Pokemon in Alola, which will very likely be Magearna. Pokemon Ultra Sun and Pokemon Ultra Moon make quite a lot of changes from their predecessors, but other areas have remained mercifully untouched. As the saying goes, you shouldn't look a gift mythical Pokemon in the mouth.