Responsive Mobile-first qr code component - html+css only

It was a struggle to understand how to adapt the text and its size. I had a multitude of problems with the 'Live site URL' of this challenge, nowhere teaches these things, and I had to learn by trial and error and I still don't know exactly what I did, nor how I did it, probably for those who have experience it's fine easy, is there any place that teaches this, or is it on the basis of trial and error?