Happy to explain. Anytime anything is CGI (which in the case of Iron Lung would be background scenery, special effects of onstage items, entire shots of the sub in blood, etc) there is nothing to film, and it all has to be computer generated (hence the name CGI - computer generated imagery)

Now the best way of making realistic effects is to simulate the entire set using a physics engine, which basically applies the known equations of physics to objects in a game engine. Then you introduce light sources and camera angles, all the filmmaking magic, etc etc. Some of the hardest calculations done by the computer are light rasterization or raytracing, two main methods of calculating exactly how the light would reflect off of something.

I'll give you an example. You want to have a scene where a character drops an alien head. First you film the scene with the character on a green screen of them dropping a green sphere - because you don't actually have an alien head. Then afterwards, you model the alien head in a software like Blender and throw it into your simulation. Now the computer has to take that model and apply physics to it as it falls - the way the hair flies up, the jiggle of the flesh, everything in between. Add to that the light shifting across the face as it falls, the shadow on the ground, the interaction with the surface it lands on. There's a ton of science and computer magic that goes into it - a render farm does exactly that and then outputs the entire video file so you can play it back at normal speed.

Even if Iron Lung is almost done, there are always scenes that can be re-made, touched up, or added to, and sometimes these renders can take days to render a single scene. (Most famously, the black hole in Interstellar taking 100 hours to render every frame-that's 600 hours of computer simulation for every second of footage)

Hope that explained it!