I do not know of a shortcut to expand or collapse all conversations/threads. The default in Outlook 2016 is to expand only a single conversation that has focus, and when you exit that conversation it automatically collapses again. Or at least that's how it's working on my Outlook 2016 that's part of Office 2016 Pro Plus, and I know I haven't tweaked any default behaviors with regard to conversation handling.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back