Source of heat in the center of Earth
depths of our planet

How deep? &quot;Deep&quot; is a relative thing. You can have 10 kilometres deep, or 5000 kilometres deep.

Super high pressure at the center of it

It's not the pressure per se that's causing the heat, it was the process of getting to the pressure when the Earth's formed. Compression creates heat. That's easy to know - just touch a bicycle air pump after filling up. There's also the issue of gravitational potential energy converting into heat once Earth was accreted. The the pressure and heat are together the result of Earth's formation. It's not that pressure itself makes the heat.

Radioactive decay

According to what we know, the three modern heat producing elements (thorium, uranium and potassium) are actually concentrated in the mantle, with an even greater concentration in the crust, which is just the top few kilometres. It's still deep for us humans, but extremely shallow when compared to the core. There is still some debate on this (see the two other answers), but my own personal opinion is that it's safe to say there's very little U Th and K in the core.
Also read my very relevant answer on the World Building website:
https://worldbuilding.stackexchange.com/a/123951/39999