When making a new core release, how do mantainers tag the core modules and update their info files?
Yes, it is an automated process, check below:


  The version string will be added by drupal.org when a release is
  created and a tarball packaged. However, if your module is not being
  hosted on the drupal.org infrastructure, you can give your module
  whatever version string makes sense (eg. see Release naming
  conventions).


NOTE: The above description is exact copy paste from Drupal.org link.