I didn't go to the gym and only do workout at home. I manage to tone my muscle and pretty obvious compare to before, just that not as bulk as those who keep pumping in the gym frequently. Here's what I understand about building muscle in layman terms so far.

Everyone do have muscle before they started workout. When we perform workout for sometimes, eventually we shred the fats that cover our muscle and that's when others can see us become more muscular.

After we shred that fats, then no matter how much we train somemore, our size remain the same. That's normal because our muscle is not getting more "supply" from protein. You see, our muscle actually shred unnecessary fats and convert fats into muscular fats, a.k.a muscle. The moment when we keep working out but muscle still stays the same is usually because we didn't consume enough protein for our body to have more meats to convert to muscle (something like that).

I'm not 100% sure, but so far my gym friends depends on protein shake or some kind of beverages to increase their muscle mass, then go to gym to pump to build better muscle. I guess it works the same way like we consume more meat and then work out to convert our meat to muscle.

Since u are going to gym, u can ask around or ask the instructor to learn more about those beverages that helps to increase muscle mass. Or, consume more meats in ur daily meal but make sure to avoid fried/oily meats because they will give unnecessary unhealthy fats that make ur body become fatter, harder to shred them off. Always go for clean boiled or steam meats and u can eat more of them