Flexbox, max-width, relative units, media query

It was really headache for me because I had to play alot around to get it like now. I would be really happy if some of you give good advices that I can use in my next projects.

I am really unsure if I did it correctly with max-width etc. I feel like I could do it all more smoother than I did in my code. Links about themes I can practice more would be amazing as well.

I wish all of you a nice day & happy coding.