Mobile-first design means building a website or app starting with the phone...

https://www.bright-bookmarks.win/mobile-first-design-means-building-apps-or-websites-to-work-best-on-phones-and

Mobile-first design means building a website or app starting with the phone experience, making it fast and easy to use on smaller screens. Buttons and menus fit your fingers perfectly, so you don't have to zoom or squint