There's a little test any dog owner can try. Push your fingers through the coat down to the skin. On one dog this is easy. On another they stop after a centimetre, even though the coat looks clean and glossy from above.
The reason is the undercoat. Many dogs wear two layers: longer guard hair on top and soft, dense wool beneath. That wool doesn't last a lifetime; it gets replaced. Many dogs grow a thicker coat in winter and drop it in spring. If you've been through this, you know how much comes out. In dogs that live mostly indoors, the change is often gentler and stretches across the whole year.
But not everything that comes loose reaches the floor. Some stays in the coat, hooks onto the next hair and becomes a knot. Knots grow into mats, and mats sit right on the skin. That can pull and irritate. In dense, long coats, matted hair is also considered one possible trigger for hot spots, those red, weeping patches that can really hurt a dog.
The tricky part is that you don't see it for a long time. The brush glides over the guard hair, the dog looks well kept, and underneath it slowly gets denser. That's why it pays to brush right down to the skin, especially behind the ears, under the front legs and on the back legs, where hair likes to collect.
You can never switch shedding off completely, and that wouldn't be healthy anyway. It's different if the coat suddenly falls out heavily or the skin turns red. That's no longer a coat change, it's a matter for the vet.