After you enable Really Simple SSL, by default a PHP redirect is activated, which is called wp 301 redirect in the plugin. If you notice your site can still be reached over http://, possibly the redirect does not work because the site is cached.
If you’re using Apache or litespeed, the best redirect method is the .htaccess redirect. As not all servers use Apache/litespeed, and not all servers support the detected .htaccess redirect rules, this is not enabled by default.
If you enable the .htaccess redirect in the settings (only apache) a redirect is activated that is executed even before the site is loaded, which is faster then the wp redirect.
Please make sure you know how to remove the .htaccess redirect if necessary. In a few cases the detected redirect does not work, resulting in a loop. You should then remove the .htaccess redirect using FTP.
On NGINX, if you want to do a redirect before WordPress is loaded like the .htaccess, you can add a redirect in the nginx.conf