I’ve applied these same principles to my own design practices. Focus on each part of the design individually like the header, for example. Look at the header independent of the rest of the site and note the exact point at which it will need to adjust visually to provide a good user experience. Don’t simply change to “mobile view” at some arbitrary break point.
Same with forms, hero areas, or any other elements in an app or on a site. He doesn’t get into specifics but that isn’t necessary because if you let the principles guide you, the solutions become clear.
You let the content and how the user interacts with it determine when and how the design needs to change. Otherwise you are designing everything to fit only 4 or so breakpoints regardless of what the element is and what content it contains. You can have unique media queries for each individual element rather than setting them globally and hoping everything works out.
In my experience, I tend to have to go in and look at individual elements and create special considerations for particular screen sizes anyway.
Does that make sense?