Do campaigns get content from the library or categories?

Technically, the content on any campaign you have created are gotten from the category you have chosen when you were creating such campaign. However, the category is a section of many sections in your library.

For example, the “inspirational quotes” category is a category under your library that contain posts relating to inspirational quotes only. If you decide to create a campaign about inspirational quotes, you must create content about inspirational quotes and add them to the “Inspirational Quotes” category under your library. Then you can go further to create a campaign about inspirational quotes by picking “Inspirational Quotes” as your category.