Giant Interiors
Jupiter and Saturn have similar structures. Both are still collapsing slowly, and the gravitational energy released makes them “shine” more heat out than they get from the Sun. It is carried out by convection. Helium is also slowly settling faster than hydrogen.
Note the putative core in Jupiter is about the size of the Earth. The temperature increases inward because the pressure is increasing. Jupiter is almost as big as a non-star can be. More than twice its mass causes the density in the body to go up faster and it actually gets more compact.