I haven’t seen much info regarding their setup, aside from the bits about Pants and Bazel.
Are those actually the upstream repositories, though? And is it known how they interact with them?
Google has Copybara [1], which allows portions of a monorepo to live outside as an entirely separate repository without the need for things like Git submodules. It supports synchronization of histories, pull requests, path and file transformations, etc.
In that sense, something like Copybara would allow them to, relatively, easily open source those bits, receive outside commits, and then sync the changes back to the monorepo.
