HTML/CSS Beginner's First Attempt at a Challenge

Thanks for anyone looking this over! I'm currently a software developer who works primarily in Java, but I have been learning front end web development in order to expand my skills/employability. I'm still learning the fundamentals so I recognize that my implementation of this challenge may be shaky.

What's the best practice way to apply CSS selectors? I found myself here using IDs sometimes, classes sometimes, and tag hierarchy sometimes, not totally sure when to use what. Are classes generally the preferred way? Or are classes only preferred when they're reusable? Some clarity on best practices here would be helpful!

I have many other questions about flexbox/why things were behaving a particular way but I think those are best answered by myself going through some solutions and trying them out.