BTW don't be flumoxed if you see if these supposed methods to do it, but they don't work. Ctrl Alt (Opt) _ / + or - / = do not work, not for me at least. One of those does strike through and one subscript btw so check edit undo to make sure nothing changed if you try these.
I've seen pages as recent as 2018 reference "Expand / Collapse Navigation" in the Menu bar, no. It's not there. Too bad CMD G doesn't at least open to show Sections, like say if that's the last view before you closed. So why in blazes do they have a key to open Notebooks? Different Notebooks are the least likely to contain anything related to what you're working on, being different umbrella categories. I don't go between Financial and Personal Journals a lot. Going between Notebooks seems the rarest navigation of all. Why is this all so bass-ackwards??
This is bloody irritating as it's something I keep thinking "surely it has it, it must be under some odd term I haven't uncovered." So I troll through the menus, search Help (useful on Mac with .01% up attempts,) find key command lists, down the Google rabbit hole with outdated pages and conflicting info to no avail, yet again. Frustrating time waster beyond words.
As for the suggestion or hitting F6 twice, if that works for you great, but I've never, ever heard of a shortcut / key command that involved repeated pressing of keys. For starters, I'm fairly sure that would not be programmable for anything customizable; why introduce that when there are plenty combinations to use? (Then again, not having this function is as ridiculous as anything else.) Also many on Mac are set up for media keys with F6 as Play / Pause toggle, so I can hear a split second of your music this way but that's it.
