Mobile First solution using CSS Flexbox and Grid

Honestly, this is my first time trying to build a component using HTML and CSS. I am not a pro-CSS either. I just know the basics. For this project I just made things work. I don't know any efficient way of doing things.

Making the component responsive was the hardest part for me because I didn't know anything about media queries. I just watched a tutorial, read about it in MDN, and made it work.

I used px for everything and don't know how em and rems work, so I could include those in my CSS. I am open to suggestions regarding this from you guys.