firmware/external/README.md

Background on Submodules

This project uses many submodules, and to build the final products, you will have to get all the submodules into place and build them in appropriate orders.

A good resource, from an unrelated project, is:

https://ghc.haskell.org/trac/ghc/wiki/WorkingConventions/Git/Submodules

And some background/super in-depth:

https://medium.com/@porteneuve/mastering-git-submodules-34c65e940407