It's come to my attention that slot of people have this misconception that whey is for muscle and mass gainers are for mass..
I don't even understand what this means but I will try to keep it simple
Whey protein usually contains high amount of protein and low fat and carbs. And this gives it a low calories number. When you are dieting or losing fat, you trend to eat less or keep your calories low. Therefore you need a source which is most most protein efficient. And this is where the protein powder comes in. A typical 3 scoops whey powder will give you 75g of protein with just 300 calories and very few days and carbs. Compare to other food sources, you might have to eat alot of eggs or chicken breast to get those numbers.
Therefore whey Doesn't give you muscle. It just helps to get your protein macros.
Mass gainers do the opposite. When you are bulking, you need high calories, high carbs and high fat diet, therefore the mass gainers have very high numbers to fulfill those macros. And in that process you will giant lots of fats as well but it's a part of the process.
If a thin guy is taking mass supplement to gain weight, then it's only because of the increased calories intake and not because mass Gainer is some sort of miracle supplement. And if someone who wants to cut is taking whey it's because he wants to keep his calories low with maximum protein intake.
In both cases you need to triangle hard so that the protein can do it's part. I hope this helps in choosing what supplement you want.