My best guess is that the file explorer will show the location of files related to the activity rather than the home directory

I see, thank you! This makes the choice of instance when signing up more significant than I thought.

I think I understand the terms you have explained, but I am still a little confused on viewing by "all", when I view "all" am I seeing posts from every instance that is federated with the one I'm on or only the communities/magazines that users on my instance have visited before?

Given that most smartphones are not more than twice as tall as they are wide, wouldn't having it in the layout you describe leave the same amount of horizontal screen space for the keyboard anyway?