Scientists believe the Earth's core is around 6,000 degrees Celsius. Where does this heat come from?

The heat in the core of the Earth has several sources including:

1 - The heat produced from the decaying of radioactive elements

2 - The heat that was originally produced during the formation and accretion of the planet itself (This heat has not yet faded)

3 - The heat produced during friction when denser material and elements sink into the planet's center.