The ground is connected at your batteries or power supplies midpoint.
The third picture at **broken link removed** might help you.
The whole lecture is at **broken link removed**
I often connect op-amp + supply to 12 V , op-amp - supply to 0V and use a voltage divider set at 50% and blocked with a capacitor as "virtual ground"
At the op's output i measure then -6 to +6 V referred to "virtual ground"
or, equivalently and 100% exchangeable 0 to +12V referred to 0V from
power supply.