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

https://eduardosnicechat.publishlane.com/posts/mobile-first-design-for-healthcare-appointment-booking-creating-seamless-accessible-experiences

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