Responsive NTF card component using plain HTML and CSS

Hello there! I got in quite a lot of trouble determining when I should use an ID and when I should use a class. How can I optimize my use of IDs and classes? Another thing that bugged me is that I had to nest three divs to create the shadow of the main card. How can I improve the creation of these shadows? I tried using box-shadow, but I could not control the width and height of the shadow.