That's very helpful and informative, appreciated. I think part of what confused me was making the incorrect assumption that they would follow standards, which in hindsight was pretty silly of me given that they've done this sort of thing before. (Also, I'd been scrutinizing
this other format instead, which I think was only adding to my confusion.)
For media-rich content that includes a fairly wide variety of media types, are there any non-proprietary formats you might recommend? I would prefer to more or less future-proof the books in this project, as they will eventually be included in a family archive that I curate and plan to hand off one day. I'm still adding to my ePub knowledge (drawing from a webdev background, which isn't always helpful, heh), so I have a ways to go before I get a handle on the options. (I should also mention that I'm working on something in Obsidian, in parallel.)
Still going to explore and play around with the two related Amazon formats ("Fixed-Layout with Image Pop-Ups or Virtual Panels" and "Fixed-Layout with Text Pop-Ups" -- is there an acronym people use for these?) further, but probably only for the sake of understanding them better.