Mobile-first design means building websites and apps starting with phones, so...

https://www.newpelican.com/articles/why-mobile-first-design-has-become-the-standard-for-modern-digital-services/

Mobile-first design means building websites and apps starting with phones, so everything loads fast and feels easy to use. Buttons and menus are big enough to tap without mistakes. Navigation is clear and simple, helping you find what you need quickly