Mobile-first design means building apps or websites with phones in mind first....

https://technivorz.com/why-do-mobile-users-miss-buttons-even-when-they-are-on-the-screen/

Mobile-first design means building apps or websites with phones in mind first. It keeps things fast, so pages load quickly even on slower connections. Buttons and menus are big and easy to tap with your finger, making everything smooth and hassle-free