I know the basics of iso, shutter speed, and aperture already.
No, you don't. I'm not trying to be rude, but thinking you know the basics might be why you're having a hard time. I think you should spend more time learning the fundamentals of these three things, and then it will make a lot more sense.
A lot of the answers to your questions are case by case.
What should white balance be? It should be what's appropriate for your video, i.e. do you want to shoot your video neutral, or you want to balance and then adjust color temp and bake in a warm or a cool look?
You don't have to put your shutter speed at 1/50, that's just a "normal" look. You could have a shutter speed of more or less, they have different (opposite) effects on the picture that get more pronounced the more you go in either direction. Your shutter speed should be appropriate for your desired look.
Why do you always shoot with your lenses wide open? There are reasons to shoot wide-open and reasons to shoot extremely stopped down, and many variations in between, each for both artistic (depth of field) and practical (amount of light) reasons.
These questions/practices just illustrate you need to spend more time shoring up your fundamentals.
sometimes the video looks dark. Do I fix this in video editing programs?
No, not if you can avoid it. This is like saying, "my levels on this singer are too low when I record, so instead of increasing my record levels, I'll just fix it in the mix". It's always better to get good quality stuff in camera than try to compensate for bad recordings in post.