I always thought Slackware [1] was the basic Linux distro which people could use to 'understand' Linux. How does Slackware compare to LFS in terms of understanding the Linux fundamentals?
LFS is utterly different. There is no installer, there is no base system, there is nothing even approaching the power of Slackware's package manager, as barebones as it seems today.
This is compile everything from source in a separate environment and put everything in the right place essentially manually*.
Of course, `make`-and-friends will usually do the actual lifting, but you're in charge of giving direction, there isn't a distro community patching packages and build-scripts to adhere to a consistent FSH or the like.