> Just curious, what is the procedure that does NOT involve Yocto? I guess a ton of shell scripts? Where can I learn it (i.e. build a Linux system for any embedded system without using Yocto or similar tools)? Is the LFS project the first place I should visit?

There are other tools in the same space like buildroot, but I would personally tend to recommend LFS to start from the fundamentals and work up, yes.