When we access to facebook page on mobile, it redirect to URL 'm.facebook.com' that m represent for mobile. Even when we access to this URL in desktop, it still return mobile UI.

As I guessed, this was for reduce CSS or JS or anythings that handle differencely display for mobile and desktop.

But I wonder there is other advantage for that? What is the keyword for this kind of technical? This must be archived by perform some actions in both back-end and front-end side. I did a lot of search but still not found any artical about this technical