The Yocto Project
The Yocto Project is a more complex beast than Buildroot. Not only can it build toolchains, bootloaders, kernels, and root filesystems, as Buildroot can, but it can generate an entire Linux distribution for you, with binary packages that can be installed at runtime.
The Yocto Project is primarily a group of recipes, similar to Buildroot packages but written using a combination of Python and shell script, and a task scheduler called BitBake that produces whatever you have configured, from the recipes.
There is plenty of online documentation at https://www.yoctoproject.org/.
The structure of the Yocto Project makes more sense if you look at the background first. Its roots are in OpenEmbedded, http://openembedded.org/ which, ...