Source of heat in the center of Earth If I read around most of comments and articles on the Internet give two main reasons for the heat found in the depths of our planet :

Super high pressure at the center of it
Radioactive decay

I need clarification on both points.
Pressure: while it's pretty clear why pressure and temperature are bound in gas (*)  this becomes unclear when matter is in solid or liquid phase. Why pushing on a solid mass would cause its thermal agitation to raise? It looks to me like most of the people takes for granted such point without proper analysis which of course involves more physics than geo-science.
Radioactivity: if I put together radioactive matter it will produce heat by radioactive decay. That's fine. Nevertheless my assumption is that if I take 100kg of any matter here at Earth surface it should contain the same percentage of radioactive isotopes of 100kg of same matter taken from the center of Earth. If this is not the case, then I ask my self why we have higher percentage of radioactive isotopes in the center of Earth rather than in its surface?
I've read somewhere that unstable isotopes are heavier and that explains why they accumulated at the center of Earth, just by gravity; but usually radioactive isotopes have just few neutrons more (or actually less!), and that make the element heavier than just 2 or 3 element next to them in the periodic table. For example Carbon-14 is lighter than Oxygen.
Can someone clarify if these two points are valid or not, and why?
(*): even if with volume and mass constant I would say you can augment pressure by increasing temperature and not the other way round as you can't augment pressure under the constraint of having mass and volume constant
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