Someone once asked...
Someone answered with kindness...
have good genetics and eat in a specific way.
Another soul answered...
lose fat. have a diet balanced on the macro nutrients, ideally prioritizing protein when otherwise. exercise--it goes back to 'lose fat'.
Another soul answered...
It's easier to recomp after being fat than to increase muscle while skinny. That's the "secret" if there ever was one. It's not genetics. It's why most weightlifters are fat. It's more efficient to build muscle along with fat.