Presently, when i have a notebook open and click back, the app/activity "exits / closes entirely. This is very different from every other app I've ever used, and even after 2 years of using Joplin between my linux desktop, tablet, and android, I still get bit by it just about every day 
I believe least surprise, and most helpful, would be to have back open the list of notebooks. Implementation wise, it would even just literally open the existing slide-out that is currently present in the app, not need for a new ~activity [android speak for each "screen" as I understand it].
I'm on android, I would expect this works the same on iOS but can't test.
PS: the only related thread I see is #2401 (make-back-button-a-general-return-button-mobile), which seems to be about the flow of back through the app. I'm just talking about the "final" back.