Back in the old days, a lot of people went through the “Linux From Scratch” process to literally build up the OS from absolute scratch. No distro, no packages, no precompiled kernel, nothing but the raw ingredients. It is a good way to really understand the fundamentals not just of Linux but of the whole computing paradigm our systems are built on. It is not as hard as it probably sounds, but it’s an investment. It takes some time and you need to put your brain in gear to actually learn.