Mobile first approach
Mobile first design is a strategy to optimize for the smallest screen resolution first, and only add or enhance experience as possible for increasing screen sizes. A mobile first approach ensures that the mobile experience is comprehensive in terms of experience and functionality, rather than a scaled down or limited version of the desktop experience. This approach is important to consider both from a visual or user interface perspective and a development approach to ensure the most important elements load the quickest.
According to Google, Responsive web design (RWD) is a setup where the server always sends the same HTML code to all devices and CSS is used to alter the rendering of the page on the device. A responsive user interface (UI) automatically adjusts for the device size. Responsive design is better for Google (due to mobile-first indexing), your customers, and development resources. A seamless viewing experience is vital to the omnichannel experience for whichever device your customers use to view your content.