How do you build muscle
"During hypertrophy, contractile elements enlarge and the extracellular matrix expands to support growth" ref.
Short answer
Lift weights that get heavier and heavier over time.
As long as you lift heavier weights over time and avoid injuries, your muscles will get bigger.
Long answer
In order to understand the underlying mechanism of hypertrophy, we need to go over a few anatomical terms.
Your muscles are made up of cells called muscle fibers. Each muscle fiber is made up of filament-like structures called myofibrils. A myofibril is made of contractile elements called sarcomeres, which contain actin and myosine. Actin and myosine "pull" on each other to create a muscular contraction. Look at the last sketch in the second image below to get a visual representation of this.
In this picture you can see how the muscle fibers make up the muscles.
In the picture below you can see what muscle fibers are made of.
Muscle growth happens when we increase the size and the amount of sarcomeres. This makes each structure above it larger. Bigger actin and myosin = bigger sarcomeres = bigger myofibrils = bigger muscle fiber = bigger muscle.