How is that even a thing?!
Genetics is unfair. Some people barely exercise, eat junk food all day, and don't take care of their skin and they still maintain a thin body with clear skin
My brother’s girlfriend is EXACTLY like that!
There are 3 different body types: ectomorphs, mesomorphs and endomorphs.
The people you’re describing are ECTOmorphs.
People with ectomorph body types are generally thin, struggle to gain weight, especially body fat. They can eat almost unlimited amounts of food, including junk food and sweets and stay looking the same. For ectomorphs, muscle gain is also more difficult than for the other 2.