Adding muscle is the ability to train your body to lift more weight. Once we accomplish that goal, we can lift more weight in more varying exercises.
Take a look at how many muscle groups we use during:
- Deadlift: hamstrings, glutes, quads, erector spinae group, forearms, shoulders, abs (when done correctly), traps, rhomboids, posterior deltoid
- Bench Press: lats,bicep, tricep, pecs (major and minor) anterior deltoid, forearms, serratus anterior
- Squats: quads, hamstrings, hip flexors, abs, tibialis anterior, soleus, calfs, quadratus lumborum (lower back)
Because of all of these different muscle groups at work, we can lift more weight during these exercises. Once we get proficient at these, then soon we can add weight to the rest of our routine.
This will lead to the muscle mass you are looking for....
....assuming you are eating right, but that's a whole other blog.