I've struggled to understand how TREE(3) works and why it grows so big. I once posted to Math.Stackexchange many years ago and got a professor to help explain it to me: https://math.stackexchange.com/questions/313134/why-is-tree3...

I also loved David Metzler's series on ridiculously big numbers: https://www.youtube.com/playlist?list=PL3A50BB9C34AB36B3