How do I add a site to a multi-site I am trying to convert my installation to a Drupal multi-site.
I have the original site (www.original.com) with a settings.php file under the sites/default folder. I want to add a new site by adding the following files.

sites/original2.com/sites.php
sites/original2.com/settings.php

I can access both URLs, but they use the default.settings.php file.
I have not changed anything in NginX as both URLs are resolved from Drupal on my docker instances.
You have the sites.php file in the wrong spot, it should be at sites/sites.php, it should not be inside any of the site directories, such as sites/original2.com/sites.php.