Keyboard shortcut to "Show Applications"

What you're describing is a standard behaviour.

• Pressing Super brings up the "Activities" overview (same as clicking "Activities" in the top-left). Pressing Super again brings you back to the desktop.
• Pressing Super+A brings up the applications list (same as clicking "Show Applications" icon in the Ubuntu dock).
  But pressing Super+A again or pressing Escape won't take you back to the desktop, it takes you to the Activities overview instead (standard behaviour from GNOME shell). Press only Super when you have the applications list open to get back to your desktop.