Saturn is more massive than Earth. Different layers represent clouds made of different materials that condense at different temperatures. Different layers represent the various regions where the temperature is cool enough for water to condense. Different gases are present at different altitudes in Jupiter's atmosphere. Winds prevent clouds from forming at some altitudes, enabling us to see deeper into the atmosphere in those places.
Clouds form randomly, as on Earth, so on average there are always several layers.
We know that Jupiter's interior is mostly hydrogen because we see the surface of the liquid hydrogen ocean. Jupiter has a low average density. Jupiter is the coldest planet in the solar system. We know that Jupiter's extra internal heat is NOT generated by the decay of radioactive elements because only stars have nuclear reactions occurring in their centers. Jupiter is heated by the tidal forces of its moons.