Responsive product preview card component using CSS grid & flexbox

This project reminded me about some css fundamentals that I learned when I started learning web development, but haven't used enough to use properly.

One of my questions has to do with the approach of this project: when I first learned html and css, I learned through building websites for desktop computers, but every other source I've seen since has argued for mobile first approach. When I started this challenge, ran into some obstacles after trying to construct my html from a mobile-first perspective rather than the desktop version. Is this something that gets easier with time and practice, or is there something fundamental that I'm missing?