You didn't phrase your question very well.
A standard, flooded lead acid, SLI battery will eventually settle to about 12.2 to 12.4 volts depending on the ambient temperature. The charging voltage applied to the battery also varies on ambient temperature, and maybe as low as 13.5 volts to as high as 14.5.
Charge and regulation strategies vary between makes, and they're too numerous to explore here. However, the GM system does incorporate the engine CPU are part of its strategy, so alternator load does play a part in the voltage you see while the engine is running. When it isn't, almost every vehicle on the road will drop to 12.2 or so given enough time.