I would imagine its a combination of different things, some gundam builders use the blue tac for wall posters to hold things in place, i could imagine on the image with the door and body some is pressed on the back to hold the door in place. I think, at the end of the day unless you plan on taking pictures like that you can achieve the same just by using tape to test fit and hold things in place while building, or like  urumomo said gravity and snug fitting pins lol