This right here: Linux From Scratch (LFS).

If your goal is to learn, there is no better way than retrieving each tool individually and compiling it from the bottom up. LFS is Gentoo ... without all the ease of use built in. More accurately, LFS is a just a set of instructions for where to go to get the code and how to compile everything you need to build a Linux system from the bottom up.

I don't live in the world of LFS or Gentoo, but my foray into LFS gave me a much greater appreciation for the distribution(s) I do use as daily drivers. I greatly appreciate my distribution maintainers.