One of the ways to help get a grasp of CSS specificity is thinking terms of “what beats what” or how strong the specificity is. Manuel Matuzovic has a helpful interactive step-by-step demo. You keep clicking the “Add selector” button, and the CSS shown (and applied to the page) changes with ever-increasingly-strong selectors applied to the body that change the background-color. At the end, it veers into not-really-selectors trickery, like using @keyframes to override things.
More specificity practice
If you enjoyed the trickery at the end, check out Francisco Dias’ A Specificity Battle!, an article we published a few years back that does a back-and-forth styling battle with nineteen steps “selecting” the same element to re-style it. CSS is cray sometimes.