QR code component using HTML/CSS flexbox

Hello, First of all, sorry for my english. Here's a few questions about this exercise :

• Is there a conventional way to organize css stylesheet ?
• Is there a simpler way or tool I can use for this exercise ? I used flex box, but I heard about grid for exemple
• Do you recommend anything that I can work on in order to get better ?

I don't expect precise answer. I'm just looking for tips about what I should do some research on.

Thanks a lot !