Looks like your outside shower is missing. A replacement should go in the holes present. Phoenix Replacement Hot / Cold Valve White - $26.99
I really can't see the hose attachment, it looks like it's just out of the frame in the right hand photo. If you connect a hose, monitor and listen as water runs. Often there's a valve that switches between "city water" and "tank fill" If that valve fails, it can allow water tank to overfill and perhaps damage it or surrounding structure.
Another modification might have been made by eliminating the switching valve and adding a gravity fill for the water tank. If you have a gravity fill port, then perhaps the water tank isn't connected to the city water port. The city water port might need a check valve to prevent the water pump from pumping water out the city hose connection. Testing will tell how many changes from the original configuration have been made.