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

https://paxtonssmartperspective.cavandoragh.org/mobile-first-vs-responsive-design-are-they-the-same-thing

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