QR Code Component - HTML and CSS

This is my first challenge here on Frontend Mentor and I am still new to learning HTML and CSS. Some areas that I would like feedback on and want to improve/become more comfortable with are:

• my use if ID instead of class. There wasn't much repetition in the project, so most elements were unique.
• an addition to the above issue, I would love some feedback on cleaner code/cutting down on repetition in CSS. Maybe there are some things that could have been put into a class, so that there was less repetition.
• my use of flexbox. Was this best practice for this challenge, any additional feedback on how I used it.
• the positioning of the card on the page. Was there a simpler/better way to position it like this?
• any other feedback on responsiveness, units, better practices, etc.