Due to the growth of muscle fibers across. Thickness, not length or fiber increase! This is called hypertrophy. A huge, multi-step and multifactorial process, in which hormones (dough, cortisol, GH), satellite cells located on the surface of muscle fibers, a number of growth factors, etc. are involved. The limit of muscle hypertrophy is an increase in fiber diameter by 2 (!) Times. That is, the fibers from the thinnest dystrophan will be no more than 2 times thinner than the athlete's fibers.
Growth also occurs due to hyperplasia - the phenomenon of an increase in the number of muscle fibers, but in relation to humans this is of little use. There is no solid research confirming the existence of this phenomenon in our country. In animals - there are examples, in humans - not proven.
There are 2 theories on the growth of the fiber itself, but as a fact, there is also no definite answer and agreement in the scientific world, too complex a process: the theory of destruction ( microtrauma) and some other, I honestly don't remember her. The first one is more popular than an example, so it's better to pick up Google and look there.
Better yet, a physiology textbook. Everything is detailed there. If you want to competently build your workouts or just know how your body works - irreplaceable reading.
Muscles are made up of muscle fibers, which are made up of protein. With an increase in the load, an enhanced construction of muscle fibers begins so that the body does not work for wear and tear. And that is why to increase muscle mass you need to consume more protein and increase physical activity