There are distros that are pre-built for you … like being given a fully functional car
(Think Ubuntu/Mint/Fedora/Suse/Peppermint/etc.)
Then there are distros that are likened to being given a bag of bits and a book and being told to build a car
(Think Arch/Slackware/etc.)
Gentoo is more like being given the tools to make your own custom parts to fill the “bag of bits” first
LFS (Linux From Scratch) is like having the concept of a shovel explained, then sent out to build your own shovel, and start digging your own iron ore to smelt and make the tools to create your own custom parts…
http://www.linuxfromscratch.org/