I'm learning to run Amazon Ads and there's something I don't understand. I get it how they work when a reader types a keyword to search for a book. But how does it work with KU users? How do they search for a book anyway? I understand they're mainly interested in those enrolled in Kindle Select, so are they given a separate database with titles free for them to read? Or do they just search like any other reader and check if it's available on KU?