In Drupal 7 and earlier, Drupal supported automatic multi-site functionality by means of different directories under the /sites/ folder. Specifically, directories there would match to the incoming domain name of a request, allowing multiple sites to be hosted in the same Drupal site. Drupal 7 also introduced a /sites/sites.php file to allow aliasing of incoming domains to specific path names.
As of Drupal 8, the multi-site functionality does not activate unless a /sites/sites.php file is present. If there is no sites.php file, then only /sites/default will be checked and no other multi-site functionality will be triggered.