In December 2018, Tiago Botelho hosted a Deep Dive (GitLab team members only:
on GitLab's Pull Repository Mirroring functionality
to share his domain specific knowledge with anyone who may work in this part of the
code base in the future. You can find the recording on YouTube,
and the slides in PDF.
Everything covered in this deep dive was accurate as of GitLab 11.6, and while specific
details may have changed since then, it should still serve as a good introduction.