About 20% of this is residual heat from planetary accretion; the remainder is attributed to past and current radioactive decay of naturally occurring isotopes.

Most of the radiogenic heating in the Earth results from the decay of the daughter nuclei in the decay chains of uranium-238 and thorium-232, and potassium-40.

https://en.wikipedia.org/wiki/Radiogenic_heating

Potassium is more or less distributed in the body (especially in soft tissues) following intake of foods. A 70-kg man contains about 126 g of potassium (0.18%), most of that is located in muscles. The daily consumption of potassium is approximately 2.5 grams. Hence the concentration of potassium-40 is nearly stable in all persons at a level of about 55 Bq/kg (3850 Bq in total), which corresponds to the annual effective dose of 0.2 mSv.

https://www.nuclear-power.com/nuclear-engineering/radiation-...

Almost none of it is from fission. Fission is a very rare natural decay mode of uranium and thorium. Most of their radioactive energy output is from ordinary non-fission radioactive decay.

No, not quite. Geothermal is powered by the accumulated heat stored in rocks from fission Uranium and other heavy atoms deep in the Earth (and other phenomena).

Geothermal hotspots do not reheat by fission or otherwise at the same speed that we extract their energy (if they did we'd be in trouble if we weren't extracting it!).

As I mentioned in another comment, build a Dyson sphere of solar panels around the Sun and it will last just as long. Build an all-Earth geothermal plant and the heat will be depleted.