Why is Earth's interior hot?

The earth's core is really hot, approaching 11,000 degrees Fahrenheit, at scientists's best estimates. But why does it contain all that heat? There are several theories to explain how that happened.
One theory looks way back to the Earth's origins. It posits that when lots of matter collided together to form the Earth, those collisions created an enormous amount of energy. All of this energy had to go somewhere, and as gravity pulled the chunks of matter together, all of this kinetic energy settled into the center of what would become Earth.
The Earth's core is also thought to contain radioactive potassium, uranium, and thorium, and these elements undergo a fission process that is thought to be the three main sources of heat in the Earth's interior through radioactive decay.
As this heat leaks out into the mantle, it creates a convection current that is the underpinning of plate tectonics and provides the energy upon which volcanoes depend.
https://www.berkeley.edu/news/media/releases/2003/12/10_heat.shtml

https://www.universetoday.com/75895/why-is-the-center-of-the-earth-hot/