Muscle growth occurs due to the growth of individual muscle fibers. Everything I write refers to the stimulus that strength training has on the muscle fibers that are capable of growing after strength training, which are those that are controlled by high-threshold motor units.

These three articles should help explain:

What is muscle growth, and how does it happen?

What causes muscle growth?

What can jumping teach us about muscle growth?

For more detail about stimulating reps, see this article:

What is training volume?