Its pretty cool you know so much! (Being serious). I have done most of my searching using youtube/google searchs/and wikipedia (I know it gets a lot of crap but its a decent non acedemic resource)). What I meant about its composition is that it isn't uniform, what the composition in one area may be completely different in another area. We havent even been able to get samples from the mantle yet (Apart from lava) even the Kola super deep borehole is 12,262 meters, the crust is 40 kilometers. On top of that, we know that the magnetic field and even gravity is different in different parts of the world, which would indicate that composition and density varies. Scishow on youtube just did a cool short about mantle blobs on a side note.

Everything i have seen regarding the earth is that we know absolutely nothing about the earth after a few miles toward the center. The sources i have seen say we think it acts like a plastic toward the bottom of the crust but we dont know a lot of the composition in most areas unless there is a volcano, but even that can be missleading because not all magma is the same composition either. And even when you hear people say 'The core is iron, thats why we have the field', well Iron has a curie point which means at 1420 F / 770 C it loses its magnetic properties. Its wild how little we know or care about the planet we live on.