ISO is simply the sensitivity of the film to light. You set your camera to match the ISO of the film so that the light meter in your camera is calibrated to the sensitivity of the film. Thats why you don't change it (we will ignore pushing film and other topics).
From there, you manage the light that hits the film by the combination of aperture and shutter speed. The bigger the aperture (smaller number!) the more light. The slower the shutter speed, the more light.
So yes, indeed you can and should change your shutter speed for each shot on the roll of film. Each shot has its own conditions, and tradeoffs for composition.
Aperture changes will vary essentially how much of the scene is in focus. Think about what you do when you are trying to better see something in the distance: you squint. Same for a camera lens, you want to 'squint' it to see better in the distance. So, in order for stuff in the distance to be in focus, you squint or make the aperture small. Think F/22 for squinting and making distant objects more in focus, vs. f/1.8 which is very wide. So, you vary the light via aperture, but there are pluses and minuses to doing so.
Shutter speed is how long the film is exposed to the light. Longer times, like 1/60th of a second, exposes the film to more light than 1/1000 of a second. Just imagine a high speed camera capturing highspeed action and each frame 'freezing' the action. The more frames, the better the 'freeze'. So, the faster the shutter speed, the more 'freezing' you will get, but the less light. So if you want to freeze action you need high speed and more light.
You simply make trade offs between shutter speed and aperture to balance how you want things to look. Maybe you are ok with more blur in distance to get faster shutter speed in lower light, or you decide distant blur is what you want so you have to manage the slower shutter speed.
Rules of thumb for speed used to be that any shot handheld needed to be at least 1/60th. But, that varied depending on your lens, as longer lenses are harder to hold still enough, so you need to have at least the inverse shutter speed to the focal distance. So a 50mm lens would be handheld at 1/60th just fine, but a 200mm lens would require at least 1/200th. While modern cameras with image stablization have changed the rule of thumb, this still applies to old film cameras.
As for stopping action , it really depends on the action, but you won't find much improvement to blur below 1/200th or so. If you really want it, go for the highest shutter speed the lighting conditions will allow.