Two proposed kinds of muscle growth, one adding contractile protein and the other adding fluid and non-contractile content inside the fiber.
The popular version says heavy low-rep work builds dense functional muscle and high-rep work builds bigger but weaker muscle. That framing is not well supported. Both rep ranges grow both components, and the split between them is far less controlled by rep range than the gym story suggests.
What is real is that strength and size do not track perfectly. A powerlifter can be stronger than a bodybuilder of the same size, mostly because of skill, leverages and neural factors rather than because their muscle is a different kind.
Program accordingly: train the goal directly. Heavy work for strength because it builds skill and recruitment, and 6 to 20 reps close to failure for size because that is what the volume evidence supports.