What is relationship between page zoom and the root element size?

Hello,

I’m trying to understand what defines the root element size. I have the impression that the HTML root element size is controlled by the page zoom: Is this correct?

If the HTML root element is controlled by the zoom of the viewport in the user agent (browser), then I could use rem units everywhere for the @media Queries. Am I on the right track with this? i.e.: the root element font-size: 16px; will change to a different value depending on the zoom and in consequence the entire site.

I ask this question because you can have very high resolution “small screens” or 1080p 24" screens. If I can control the size of the root element I have to worry less about the site responsiveness, right?

Bonus question: what is the difference between Ctrl + “+/-”, Ctrl + mouse Scroll and zooming with two fingers in a touch screen in a browser? I’ve found very few pages that behave the same while zooming with any method.