Power Supply - Shorting a cable creates a ground?
You can pick anything you want and call it ground.

This power supply box has two power supplies in it.
Shorting the + of one to the - of the other puts the two power supplies in series. That gives you a so-called "split" power supply.

You can call the middle connection ground and then you have +15V, 0V and -15V wires.

Or you can call the right connection ground, and then you have +30V, +15V and 0V wires.

Or you can call the left connection ground, and you get 0V, -15V and -30V wires.

Note - some dual power supplies have the - of both supplies connected, inside the box. In that case, shorting the + of one to the - of the other would short out one of the supplies. This one doesn't have them connected.