What URIs are accessed by drush? What URIs are accessed by drush up? Is it drupal.org? Are there any other URIs?
I have tried using drush up --debug, but I can't seem to find a list of URIs. I have also googled it using the key words drush access URL and drush access URI, among others.
If you are referring to drush dl, then by default Drush downloads from drupal.org, if you don't tell Drush to use another URL.
For example, if you use drush dl views --source="updates.modules.org/history", Drush would probably download the Views module from modules.org, instead of drupal.org. From where the module would be downloaded depends from what updates.modules.org/history/views/7.x (for Drupal 7) or updates.modules.org/history/views/8.x (for Drupal 8) returns.

If you look at what https://updates.drupal.org/release-history/views/7.x retuns, you will notice a line like &lt;download_link&gt;https://ftp.drupal.org/files/projects/views-7.x-3.14.tar.gz&lt;/download_link&gt;, which is telling to Drush to download the Views module version 7.x-3.14 from https://ftp.drupal.org/files/projects/views-7.x-3.14.tar.gz.