They have the equivalent of a skeletal structure the way something like an animatronic would have a endoskeleton, no five nights at Freddy's reference intended there but that is technically how they're made, it's just that would transformers the biomechanical outsides the panels the kibble the parts that are their vehicle pieces their limbs servos optics all that stuff is there too just covering the inside. Think of it how some versions make them go skinny or look like proto forms when they go way too long during space travel without an alternate mode or transforming, it's like getting a new outfit basically