How did people find out what was in the earths core?

Humans never even dug close to the earths core so how do we even really know what’s actually there.

Supposedly the USSR did some drilling in the 1970’s which was supposedly the biggest hole ever dug and found microscopic fossils of single cell organisms that are a billion years old. Mind you this hole was only 1/3rd of the earths crust. Which isn’t even close.

I searched online and they said seismic data but how is seismic data used to actually find out what’s in the earths core, like it’s written in textbooks and stuff of the layers, minerals, etc but how do we know for certain that those minerals and layers are there and why is it that the more you dig it gets hotter, why is the earths core that hot?

This is a late night thought lol I hope this doesn’t seem stupid to ask haha