What happens to the body during crash diets?

I know that it’s been shown that when people do crash diets it’s ineffective because they regain the weight. Defining crash diets here as someone losing an aggressive amount of weight quickly, like 2-3% per week.

But what actually is going on when someone who weighs 200 lb loses 20 lb in 2 weeks and then regains it when they resume their prior eating habits? Do they lose fat and then just quickly regain it when they eat more? Or is it just water/glycogen that was lost and replaced?

This is not necessarily true, as the proportion of people regaining weight after dieting is about the same with rapid vs slow weight loss.

the majority of the weight lost in 2 weeks is likely to be water. The greater the energy deficit, the more actual fat (and muscle to some extent) is likely to be lost as well. In many, this tends to increase appetite, suppress feelings of satiety, and reduce metabolic rate to compensate for the loss. This also happens with extended weight loss.