Keyboard shortcut to open Ubuntu file manager?

It is under System Settings -> Keyboard -> Shortcuts -> Launchers, and by default on my system it is set to Explorer key (whatever that means), but personally i've set it to Super + E , since this is the same as Window's shortcut, which is what I used to use a lot year and half ago when I still was a Windows user. Just click on the area where the description for key is, it will say new accelerator and press keys you want to set

And if you want to create a shortcut to open some directory other than home, go to Custom shortcuts, and set a new shortcut for nautilus /path/to/dir command. For instance, nautilus /usr/share/applications