How to go about basic HTML creation in regards to CSS

Hey guys, I have a question that’s been bugging me and serving as a big roadblock in my understanding of making basic websites with HTML and CSS. How does one know when to create a class or a group of classes for an element? For example, let’s say you make a simple design and begin creating the structure (HTML); how would you know when to begin giving each element (div or otherwise) a class of card or card / big card? It always confuses me when I see an HTML document that goes in depth with many elements attached to classes. Any help on how I could break it down better or understand it would be greatly appreciated.