I’m sorry, but if the movie has already told you, on more than one occasion, that poop exists, that should be enough information for you (it’s a little too much for me, but it doesn’t bother me). It should really go without saying that if poop exists, it has to go some place. You’re going to question that such a place exists until the movies show it to you or explicitly work it into a line of dialogue? Then what’s your alternative explanation for what happens to the poop? It disappears seconds after you excrete it, the way Yoda and Ben disappeared after they died?