Mobile-first design means building websites or apps starting with the phone...

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

Mobile-first design means building websites or apps starting with the phone experience. It’s all about fast loading times, easy-to-tap buttons, and simple menus that make getting things done quick and painless on a small screen