QR code component / HTML & CSS

Hi there! :) Thanks for your time! I really appreciate it.

What did you find difficult while building the project?

This is now my second challenge I have done on this page, and I am very happy so far. This challenge felt easy and finished quickly. It felt nice to build it, but I am thinking a lot of my code of how readable it is or if some tags are unnecessary or not.

Which areas of your code are you unsure of?

Three things are a big concern for me at this challenge:

1. How to know which unit should be used? I am unsure about when to use px, % or go to vh, vw, em and so on.
2. How and when to comment the right way? At my first challenge, I commented almost everything. This time only the Main elements. Is there a plan on how to do it right?
3. I am using <main> and <section> tag just because I know there should be at least one main tag in a page but for this challenge it felt like I don't need the main or section tag, but I did it because I wasn't sure about it.

Thanks in advance for your Feedback and your time you invest to read this! I wish you a wonderful day.