qr-code page / css positioning/relative/absolute/fixed

So really the only thing that I found difficult was the positioning. Im just starting out so I really didn't want to use flex/grid, just to really get the hang of positioning relative/absolute/fixed. To get the div right in the middle of the page was difficult for me I found a solution on stack overflow (https://stackoverflow.com/questions/9862167/positioning-div-element-at-center-of-screen), its the third solution down it work for me but I'm really not sure about the reasoning/why it works. So I would really appreciate the feedback, maybe explain why it works. But this was really great for my first challenge! Thanks! -Michelle