Overly simplified, but here it goes:
Muscle building requires:
a) An stimulus of both the muscles and the nerves (lifting)
b) Progressive overload and tension -its not just move the muscle, but actually generate enough stress to overload it (sarcoplasmic and myofibrillar hypertrophy). Muscle « grows » because when you lift, you break or damage a little bit of your muscle fibers.
c) Your body then needs nutrients and Amino Acids to repair your damaged muscle fibers. You get the Amino Acids (BCAA’s) from supplements and proteins. See them as the building blocks of your body.
d) Muscle building is a very energy demanding process and will require an excess of calories to actually happen. Your body will use the calories you feed it first for its essential needs, and will put muscle building at the last of its process, (as muscle building is really not essential for survival), hence, why you have to eat above maintenance.
e) You gain Muscle over Fat, as long as you eat just enough calories over your TDEE. You will still gain fat if you don’t really watch your Macros and overeat: