If we simplify the stimulus for muscle growth into the local and global. Local being everything that happens within the muscle, as well as its receptors. Global being factors outside the muscle itself such as growth hormone release from the pituitary, circulating testosterone, and systemic counter regulators like cortisol and inflammation.
Of the two (local and global), the local is the much more powerful stimulus, and in many ways necessary for the muscle to actually take advantage of any global stimulus. This is why simply having elevated levels of hormones, naturally or artificially, still requires training for muscle growth.
Locally muscles respond to tension, mechanical damage, and biochemical stress generated during training. Each of these stimuli has a threshold, that when reached will stimulate a pathway that ultimately leads to hypertrophy. In a beginner, the threshold is incredibly low, which is why even the worst program, with terrible execution can result in beginner or noob gains. As you train, the threshold for stimulating muscle growth increases because the muscle adapts, and becomes more efficient as well.