Meta does have a monorepo but their open source stuff lives outside it. Or at least it did when I worked on PyTorch (2019). I did all my work in the separate open-source PyTorch repo and then commits got mirrored back to the monorepo by some automated process.

You could also build and run it using completely standard tools; you didn’t need to download random internal source control software etc. like you do for e.g. Chromium.