In my experience the Google tool is just not smart enough to figure out clever CSS or other techniques you may have used to make your screen mobile-friendly. Just fro example, I have had tables which remove columns depending on the screen width and even used bootstraps "table-responsive", but until I put in a pointless div to style "overflow-x:auto;" it reckoned the content is wider than the screen. Similarly, I drop some clickable elements on a small form factor but Google still reckons clickable elements are too close together even though they don't show on a mobile device. Actually, It's just a dumb tool, so need to find workarounds or dumb down the content.
Basically, Google’s algorithms should be able to automatically detect this setup if all Googlebot user agents are allowed to crawl the page and its assets (CSS, JavaScript, and images). As per Google standard, Pages optimised for a variety of devices must include a meta viewport tag in the head of the document. A meta viewport tag gives the browser instructions on how to control the page's dimensions and scaling. To attempt to provide the best experience, mobile browsers render the page at a desktop screen width (usually about 980px, though this varies across devices), and then try to make the content look better by increasing font sizes and scaling the content to fit the screen. Font sizes may appear inconsistent to users, who may have to double-tap or pinch-to-zoom in order to see and interact with the content.