Q: I need to know how to transfer purchased items in iTunes to my library

I have been trying to restore my iPad mini through my MacBookPro, but I get a window saying this: "There are purchased items on the iPad that have not been transferred to your iTunes library. If you restore this iPad, these items will be erased and you will have to purchase them again. Are you sure you want to continue?"

I went to my iTunes  and opened my music, but I cannot figure out how to transfer my purchased items to my Library. I always thought that every purchase was placed automatically in the library

I do not want to purchase these items again, how do I work it out??

Thanks!!