Drupal.org, like many non-trivial sites, has a site-specific module and theme. The module, drupalorg is maintained like any other module. The theme, Bluecheese, is in our private Bazaar repository, so users will know when they are on the official Drupal.org. Implementation notes for each are on the project pages.
Drupal.org is made up of a few independent Drupal installations, like groups.drupal.org and localize.drupal.org. This breaks up a complex site into smaller sets of functionality that can be upgraded, changed, and maintained by independent teams at their own pace. We do single sign-on with Bakery and strive to have the sites feel like one site for users.
Key functionality on Drupal.org is project management for modules, themes, and install profiles. project provides projects, issue tracking, and release management. cvslog integrates with version control, but will be replaced.
Search is done with Solr. UI customizations go in the drupalorg_search module, part of drupalorg.
A variety of other modules are used. The most current list is always what is checked into sites/all/modules in the Bazaar repository. More can be added, after careful functionality and security review.
Improvements to all of these are done with the usual patch-and-review workflow.
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