Once upon a time there were just PCs and a 12 inch screen was pretty good. Then there were laptops, and notepads, and then mobile phones with snall screens, then with larger screens, then tablets and phablets, and ... . Ugly truth #1 is that your site may be browsed by any number of devices and screens sizes, it is not merely mobile and non mobile.
Not all screens are the same, and attempting to show the same content to all screens makes some users of some screens frustrated. Ugly truth #2 is that it is your site's problem to deal with the unpredictable variety of screen sizes.
The way to deal with these ugly truths leads to ugly truth #3 - you have to give up the idea that you can exactly control how your site looks. You can try to exactly position each element, decide just how much spacing you want after the sidebar,etc, but whatever you decide will most likely work only for the screen size you were using, and when viewed from a very different screen size may look quite different. Instead you have to let the browser move things around to try to show all that needs to be shown.
Ugly truth #4 is that you simply cannot show as much on a small screen as on a larger one. Folks just do not love your site so much as to scroll endlessly reading your lovely content.