Desktop first using plain HTML and CSS

I tried to implement the mobile design as well, using media queries in CSS. When I look up the URL on my phone, however, all I get is the desktop version.

Even though my phone's screen is way smaller than my PC's one, obviously, my phone's resolution is still 1920 x 1080 px. So I wish I had a way to adjust the site's layout based on actual screen size rather than resolution.