QR Code Component with Flexbox

Hi, friends!

I'm very new to HTML and CSS but decided to try out Frontend Mentor. I've been following a Udemy web development course, and this is the first time I've really strayed away from it. I definitely felt the gaps in my knowledge. Until I did this project, I didn't really understand nesting in CSS. I think this has given me a better idea. Also, centering items within the body of the page is particularly hard for me. It took a lot of Googling to figure it out. Any pointers on how to remember what to do?

I'm super eager to learn more, so please let me know if you have any feedback at all!

P.S., I wasn't sure if that was a drop shadow or compression artifacts around the card... so I added a little shadow.