That depends, in some people it's resistance first (essentially prediabetes), in others it's obesity first. Some others develop insulin resistance when even moderately obese, there are a few who never develop it despite being extremely overweight.

The lower insulin resistance, the better the effect of diet, usually.

If that were true, weight loss surgery wouldn’t help. It takes away their ability to eat too much at once by reducing the size of their stomach. - it doesn’t change their insulin levels.

If you watch the show, most of them overeat as a coping mechanism and part of the treatment is psychological help.