Oh, I didn't know that shortcut for minimizing. Is there a reason the button is hidden by default?

I never really understood how to efficiently use virtual desktops or what their benefits are compared to one desktop. Would you mind to explain?

Well, I would imagine that is because you generally only need extensions on KDE for niche things, while GNOME needs extensions for more 'basic' things. Obviously you don't need an extension for a system tray if one already exists by default.