How was this movie made?

I still don´t get it, how did they manage to do some of the things in this one?

- All of the scenes with the fly using objects with it´s tiny hands, especially when it´s eating *beep* in the toilet. I also wonder how they managed to get him to talk.

- Heidis massacre, of course they didn´t use live rounds, simply because if that would have been the case, the production would have been cancelled due to the safety.

- The Vietnam scene, just look at all things going on here, and then the russian roulette scene, how was it made?? I just don´t get it.

- Also, the scene when some small creatures has invaded Sids lounge and makes a complete mess. How was it made for example when one of them is puking or something similar?

- Arthur the worm, how was his movements controlled?

- The scene with the silver fish who gets eaten up by Bletch, how did they made him talk?

- Seymour, the son of Sid, when we see him the first time in the baby cart, how did they got him to move and sound?

I do have a theory for the scenes with the fly, wasn´t it just that someone moved it´s hands and then the editor or editors just removed the persons hands?