When I add a class to one element, it adds it to the others

So when learning how to edit items individually, I was told to create a combo class for each element, but when i added a combo class to one element it would add the class to all the elements.
As an example, I added the class “mop” to an image on the landscape mobile breakpoint so that it would only change that instance of the image, but the image on the desktop breakpoint also had the “mop” class, so when any changes were applied to both.