Qt for Python (PySide/Shiboken)

I am trying to understand a very specific concept, which I am currently going deep down the rabithole. If one would consider a QComboBox. And this QComboBox is connected via a QDataWidgetMapper. Under the hood there is likely some QItemDelegate that does the magic where the model can have a plain string, and the QComboBox would correctly select the right item. Where / when does this text to currentItemIndex happen? What is basically the slot that is connected? Bigger question: what should a widget implement so it would not require custom QItemDelegates in the code by the user?