How to detect mobile device like Zara.com?

I did some research, and am aware of there are 2 ways to detect a mobile website, either by check user-agent, or by screen resolution. Though it seems that zara.com does not use 1 of those 2. For instance, open zara.com with chrome debugger mobile mode would not return mobile site, or "Request desktop site" on iOS would not return desktop site.

So I am really curious, how does Zara do it?