There's a nearly-negligible amount of heat being created from simple radioactive decay, but essentially all of the heat is just leftover from the formation of the planet.
Which is just nuts to think about. The core is that hot and it has been for billions of years. Incredible.
Emitting heat technically. Imagine rocks warmed by a fire or those in a sauna, they cool off very, very slowly. At earths core is a giant chunk of super compressed and super heated lead. I don't recall if the core is the size of the moon, but you could imagine a solid ball of lead for your sauna that is the size of the moon and so hot it would melt if it were not under extreme pressure.
Which is just nuts to think about. The core is that hot and it has been for billions of years. Incredible.