How do they seal it to stop mud getting in those gears? A huge rubber boot? Some kind of set of sliding metal plates? Seems hard to design something sufficie

How do they seal it to stop mud getting in those gears? A huge rubber boot? Some kind of set of sliding metal plates? Seems hard to design something sufficiently robust to last the lifetime of the car while not letting I. A single drop of water or grain of sand (which would make quick work of destroying these gears)