Linux from Scratch
Everything is built from scratch.
First, you need some kind of host Linux system, which then is used to compile and install everything to target environment.
It is quite a huge undertaking, needs lots of time and patience. Basic LFS book is around 280 pages, and it will just give Linux terminal - you will need to go Beyond Linux from Scratch to actually get desktop environment running.
I have installed it twice, as a learning experience.
It is not a distribution, but when you build your own Linux from Scratch, it will become your own distribution - thus it is distribution!