Yag is available on its Steam store page
Yag content is quite storage heavy, so in order to ease your storage management, all the Yag content is managed through free DLCs.
Yag content is part of Yag: when you buy Yag, you get all its content, so DLCs are all free. The only purpose of using DLCs is to allow you to install and use only the content you need.
DLCs are organized by themes, you can grab them on their respective Steam pages.