Contrary to the traditional approach of starting with a desktop site and then adapting it to smaller screens, a “mobile-first” approach involves designing a desktop site starting with the mobile version, which is then adapted to larger screens. A mobile-first approach generally means building your website with your mobile users in mind, with the main goal of improving these mobile users’ experience.
Since Google made ‘mobile friendliness’ its ranking factor in 2016 and later the mobile index its primary index, the mobile first approach gained momentum, because the mobile website version is crawled instead of the desktop version.
The concept of “mobile-only” refers to a strategy which is built uniquely around mobile devices. A “mobile-only” strategy might mean, for example, that a company offers its products and/or services uniquely via a dedicated mobile application.
The majority of the time, we see mobile-only strategies with companies, products and services related particularly to mobility and social meetups, where actions are carried out exclusively or nearly exclusively via a mobile application.
(Source: AT INTERNET)