extdir handles both the extension directory and also has a Drush command to scan/update known extensions for new releases.
This runs from Jenkins:
To run manually (from www-prod-2):
sudo -i -u aegir
drush @civicrm.org extdir-scan
Some documentation in the dev book:
The code does not seem to have its own git repository. It is part of the civicrm-org-platform repo:
Service that runs on top of the Drupal extdir, to add more information about extensions.