In this era as more and more website visitors are using smaller devices, primarily mobile phones it is ever more important to get the mobile experience right. When you’re a site owner or prospective site owner, you need to think about this factor and prioritize it. There are still many agencies and developers who will design for the desktop first and then made adaptions for mobile.
Designing mobile-first means if you are doing mockups whether it’s with Illustrator, Figma or some other tool make sure you create the mobile view first. That’s exactly what mobile-first means, is start there. Then of course, you can start to think about how will our layout change as we have more real estate to work with? This usually means widening the entire space, even if you plan to “box” the layout such as limiting the width to 1160 pixels or some other common break-point.