How does Web Developer determine the class name when it's not in the .css?
It's reading the HTML as assembled from all the php files and the database and then rendered in your browser.

The IDs and classes are already there in the code for your use. But quite a few of them are not used in the default stylesheet. So anytime you see a class or id in the code that you'd like to use, you can add it to the stylesheet and set it up as you wish.

Rob