Actually, all the heat in the interior is due to radioactivity, right? There is no way solar radiation gets past the first few feet of the earth, and nor past the water envelope of the crust.
Radioactivity, in part, but also depth and pressure as well as convection to spread the heat around.
The earth’s interior heat is due to global warming and it’s millions of degrees! https://www.youtube.com/watch?v=kGV7Dr2iDvU