Mobile-first design means building apps and sites with phones in mind first....

https://wiki-saloon.win/index.php/Why_Do_Crowded_Mobile_Layouts_Cause_More_Mistakes%3F

Mobile-first design means building apps and sites with phones in mind first. This approach makes sure everything loads quickly, buttons and menus are easy to tap with your fingers, and you can find what you need without digging through clutter