What SAB is expecting: Keeping in mind there is some flexibility in SAB, the main way it is intended to work is one .sfm file per Bible book (e.g. Luke, John) that contains multiple chapters with \c 1…\c 2 etc, and then multiple sound files that correspond to each chapter. So one text file per book, but multiple sound files per book. Although you could conceivably add one text file per chapter, SAB is not expecting this and the table of contents will not be very user-friendly. If you are using .sfm files this will be very easy.
Page vs chapter: You’ll see when you make an app with \c chapters and open it on your phone it has one chapter per very long page that you scroll down, so you can use chapter and page somewhat interchangeably in this context. Then if you have multiple versions, in other words multiple Bibles, you can have Book Collections. For example if you had ESV and NIV Bibles in your app, in SAB parlance that would be two Book Collections. But in those book collections you would have each book as its own file - see the first screenshot above where for example GEN is 01GenwolKYG.sfm - that text file is straight from Paratext and has a bunch of \c chapters in it.
Portions vs Whole Book: I have one app I’m working on that is a revision of an existing work. The translators are only ready to test chapters 1-3 of Genesis, although they are still working on the rest of that book. When I make their testing app, I refresh the sfm file through send/receive in Paratext, Update from Original Source in SAB, and then make the app. But to make sure I get just chapters 1-3, I have put “1-3” in the Portions box. If I put Whole Book, it would put all of Genesis. When they add say chs 5-6 but still want to keep chapter 4 out of it, I could do “1-3, 5-6”.
Hopefully that gets things a bit clearer - come back if more questions arise though.