Mobile-first design means building apps and websites to work great on phones...

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

Mobile-first design means building apps and websites to work great on phones first. It focuses on speed so pages load fast, a touch-friendly layout that’s easy to tap, and simple navigation that helps you find what you need without hassle