When using CMD-Tab, how to re-open the app's main window?
There exists a shortcut for that, though it requires certain dexterity:

Command-Tab to the app, but then, before releasing Command, hold Option.

If you then release Command while still holding Option, the window should pop open.

It's just like clicking in the app icon in the Dock (if there are no windows open, one opens; if there is a minimized window, it appears again).