If you hit this page with a browser, that the server thinks is a mobile browser then you will be redirected to the mobile version of this page. The detection of the redirection is done by looking at the user-agent header in the request.
Note: the server does not have particularly good user agent detection so it will not work for all mobile user agents. It is deliberately buggy.
The user agent page is at /styled/redirect/mobile/user-agent-mobile-test
If you are automating, then you can either detect you were redirect by looking at the title, or the url.
In the real world, you will normally find a subdomain being used e.g. instead of https://eviltester.com it would be https://m.eviltester.com
Note: This is a pretty old fashioned way of handling mobile devices. Most websites now are responsive, and will work on both desktop and mobile, and so no separate mobile pages, or subdomain would be used.