Content can be added to a single class or multiple classes at once.

You can add content to multiple classes at once from the Library tab in the top menu.

Select content from any available category in the Library by clicking the Select button under a content item.  

Some content types also have additional information, such as a preview video, overview, learning objectives, keywords, or description.  For content where additional info is available, the i button below the image will open a pop-up window where you can learn more about the content item.

When you have selected your content, click the Add to Classes button at the top right, choose your class(es) from the list, then click Apply to assign the selected content to the selected class(es).