[nseggs]

To be clear, that first one is pretty outdated, and while Yosemite was not day one, it was over 6 years ago now.

>maximize a window

Control-Command-F: Use the app in full screen, if supported by the app.

>file manager

Option-Command-Space brings up a Finder window from anywhere I believe, but defaults it to search mode, so you may have to chain the shortcut for a particular directory or Go to Folder after that.

I'm guessing the Activity Monitor thing is more that there is not a dedicated shortcut for launching any single app in macOS (that I can think of).

[giantrobot]

Command + Shift + g in Finder brings up a "Go To Folder" window so you can type in a folder and go directly to it. Bonus 1: it supports tab completion. Bonus 2: this also works in file open dialogs.