QR code component with flexbox

This is the first project I made and published.

What did you find it difficult while building the project?

I had problems with setting the right height and width and centralizing objects.

Which areas of your code are you unsure of?

I didn't use Media query so I'm not sure if this is the expected result, the part while I centralize the objects and the size of the body.

Do you have any questions about best practices?

when you deal with a specific element it's better to cascade like body > section > a or use a class?

If anyone has good material on centralizing and tips on margins I would appreciate.