They say:
1. Mostly from the formation of the Earth. Leftover heat still exists and is well insulated... which just seems bonkers it can stay hot for so long.
2. Friction, but not spinning... they say it's from heavier material sinking to the center.
3. Radioactive materials. No idea how much, no good way to calculate.
https://www.scientificamerican.com/article/why-is-the-earths-core-so/
"The amount of heat that can arise through simple accretionary processes, bringing small bodies together to form the proto-earth, is large: on the order of 10,000 kelvins (about 18,000 degrees Farhenheit). The crucial issue is how much of that energy was deposited into the growing earth and how much was reradiated into space. Indeed, the currently accepted idea for how the moon was formed involves the impact or accretion of a Mars-size object with or by the proto-earth. When two objects of this size collide, large amounts of heat are generated, of which quite a lot is retained. This single episode could have largely melted the outermost several thousand kilometers of the planet.""Additionally, descent of the dense iron-rich material that makes up the core of the planet to the center would produce heating on the order of 2,000 kelvins (about 3,000 degrees F). The magnitude of the third main source of heat--radioactive heating--is uncertain. The precise abundances of radioactive elements (primarily potassium, uranium and thorium) are poorly known in the deep earth."
http://cansay15physics.blogspot.com/ had this nice graphic, which shows the core spinning, which still makes me wonder about friction there... the whole Earth spinning vs. the core, etc.:
And this has cool info on the crust and stuff: http://geologylearn.blogspot.com/2016/01/what-are-earth-layers-made-of.html
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