As I understand it, Desktop rendering is full-scale, and the viewport parameter is only used by smaller screens to reduce the size, TO FIT.
You interpretation is not correct. The meta viewport tag when set to 1 tells the browser to maintain the size ratio 1:1 that is for whatever screen width 100% width == full size of that screen. So if you have small screen on your device and your website is designed to be say 1000 pixels at full size (desktop) , the viewport == 1 setting will prevent your browser from reducing the size to fit elements onto the smaller screen. Assuming a phone size 333px (for simple calcs) This means that for 1000px wide body width only 1/3 of the page would be visible. Without it, it would take a 1000px and reduce it to 333px, the entire page would visible but it would be shrunk by 1/3 making the elements unreadable.
The viewport tag is one minor component required to a make a page responsive. The main means of achieving responsiveness is by using Media queries in your CSS. What these do is assign different styling to elements on the page as a function of screen size. This is where the concept of mobile first comes into play. A mobile first layout will be such that the layout without any media queries will display correctly. This generally entails limiting the width of your page elements to 320 or 360 px, and arranging the content in mostly stacked configuration, one very tall column. Then as screen sizes grow, you can split up the column into 2 or 3 wide.
There is an opposite strategy, where one can layout the content for desktop and then collapse elements into narrower formats and eventually into a single column. This was preferred practice when this technique was first put use (circa 2013?) . In addition to resizing and styling many designers would also hide content on for small devices. Back then most web traffic came from desktop. Now it is the opposite.
In 2019 Google is crawling and indexing websites with a mobile crawler and using the mobile "image" as the gauge on which to rank the website. Hide content on mobile is no longer a good practice.
Furthermore, I believe that simple collapsing content into a single column is not great for user experience and certainly sucks for displaying ads. Simply because the content will take up the entire view and thus ads will rarely be seen. The concept of "Above the Fold" is nearly meaningless on a mobile screen. User's scroll, and scroll fast they will often scroll past the ATF content before it has time to load, specailly ads. Remember that just because an impression is recorded doesn't mean that the user actually sees the ad. Users can't click ads without seeing them. And if the can or do then you have bigger issues than low RPM.
I believe that one must optimize ones content for mobile. Ads must be placed a close as possible to key content and the content should be displayed in such a way that it can be consumed with minimal scrolling. I like to use carousels, but you content will dictate how thing should be arranged.
My Alexa bounce rate has been 88% for 16 years.
Time on site is less than 2 minutes.
The visitor visits, get what they want, and leave.
You overall stats, bounce rate, time on site etc.. is of little importance. For each metric, you need to look at the difference between mobile users and desktop users. In terms of AdSense arguably the most important metric is AVV Active View Viewable. Check out the difference between mobile and desktop for AVV, you will be shocked.
Google still makes us calculate EPC, a PITA.
In AdSense CPC == EPC.
I disagree with Robzilla, it is absolutely correct to use RPM over CPC or CTR, as RPM is a standardized metric making much easier to compare in a like for like manner. CTR & CPC only covers a portion of the total impressions and are very much biased by the number of volume of impressions a CPC of $2.00 for 10 impressions is not the same as CPC of $0.20 for 10k impressions.
