Mobile first site using HTML, CSS, and Flexbox

Hi, everyone. I'm pretty new to HTML and CSS and this is the first thing I've ever coded after watching a short, basic HTML and CSS guide, so go a bit easy on me.

Questions:

Is there anything I could've done more efficiently/succinct?

I found myself confused on when to use padding and when to use margin when there isn't a border to create a difference in appearance. What's standard?

I didn't know what media query min/max-width to use for when to switch between mobile and desktop and instead chose the px for when the image started to be cut off. Do you have any reccomendations?