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

https://titusssplendidperspective.almoheet-travel.com/what-industries-pushed-mobile-first-design-forward-the-fastest

Mobile-first design means building a website or app starting with the smallest screen—your phone. It focuses on speed, so pages load quickly, and uses big, easy-to-tap buttons for smooth navigation with your fingers