Hey Cyrill. Your layout fits nicely in a grid (content lines up in both the x and y axis), so I'd recommend using CSS Grid for that.
Here's your code refactored into a grid layout: https://codepen.io/geddski/pen/LYZNPVO?editors=1100
Something I did early on (and still do) is pick a cool looking UI (video games often have amazing ones) and try to recreate it with HTML & CSS. You'll learn a ton that way. Don't get hung up on doing it "right" at first. Just focus on getting it to look right at first. Best practices will come...