Many people have the same goal. They want to build muscle and lose fat.
However, to optimize one or the other, it is best to have two seasons for each goal. While it is possible to simultaneously gain muscle while leaning out, the rate of muscle growth is very slow in comparison to a program structured for just that in mind. If your protein intake is adequate, muscle tissue can be synthesized in a deficit. When you’re in a deficit, your body is burning more calories than you are consuming, so placing the expectation for muscle growth on top of that is quite demanding.
Your body needs protein + energy (glycogen + triglycerides) in other words – carbs and fats to synthesize muscle tissue which costs energy in itself. Having your body in a surplus of energy creates a better opportunity for muscle growth. Many might be fearful of gaining fat during a building phase but as long as your intake is controlled by numbers you can minimize fat gain by adjusting your macros/training as you progress.
In turn, if you go to implement a cut to lose body fat, your body needs to be at a deficit in order for your fat stores to metabolize. This means you will have less stored energy making it harder for muscle growth to occur.
During a cut, you would also be increasing cardio to aid in fat loss, this would be counterproductive to gaining muscle mass and therefore hinder that process. For competitors, implementing two different seasons for these processes will be the best approach for bringing your true potential to the stage. Like many things, it takes hard work and patience to achieve your best physique. Changes in body composition during any program will also be dependent upon training method/volume, sleep, stress, water intake, and macronutrient ratios and timing.