Even in my original comment i said there are non js ways. But what i primarly meant here is, there are differnt “ideas” of implementing a mobile site, some just change element sizes, and make them vertical viewing friendly, but others do change quite a lot, for example, remove most clickable items from the header-bar or title bar, and move them to a kebab/hamburger menu. And I know even this particular example can be done in pure css, but I still feel my statement is not simply wrong.

Also I am not a front end dev, so I am sorry if I get stuff wrong.

That may be the case, but i have seen websites use js for it. Another thing is, what version of css does wikipedia target, as ye older verisons might not have that.

me too, i was simplifying it. I have a global js disable rule, and whitelist a shorter list

usually it is just a redirection. One of the reasons wiki does this is that their stack is more older device friendly (for the most part, you can use wikipedia perfectly fine without any js), and having adaptive view usually requires js (there are some other ways too), but wiki is constrained. So when browser recieves a request from a mobile user agent, they just redirect to mobile site.