The biggest issue you’ll run into in making a website that effectively scales down from desktop proportions to mobile is information density and features.
For instance, Netflix’s app is remarkably consistent in design across mobile and web. However, this is pretty easy because the overall feature set is limited and scalable.
For example: can you effectively have a repeating group with 6 columns of data look good at 400px wide - likely not. So you either hide certain data fields as the page gets smaller or have a mobile-friendly version of the group). (In Netflix’s case, they only show the image of the movie and the title, then you click in for supplemental detail).
When you don’t want to have mobile versions of pages, you can create “breakpoints” using the page width. (Ie. when current page’s width is 400px, then reduce the text size to X). More context in the forum post below.