This plugin is relying just on your wp-config.php file. It uses smtp connection details you will set up there to configure WordPress to relay all outgoing emails through that email server.
Background
There are plenty of similar type of plugins out there. Most of them are not focused on relaying emails to an SMTP server, but rather supporting specific mail services.
But the main incentive for this was the fact that the official WordPress docker image doesn’t have support for sending out emails (eg. “forgot password”).