That is mostly correct. The iFly GPS software will use the stronger GPS signal, whether the internal device GPS or External GPS source (ADS-B). There is no setting to change.
Typically, the ADS-B GPS antenna is placed in a manner that promotes a stronger GPS signal, thus it's signal is typically used. If you see multiple green bars in the GPS Status instrument, it's using the internal GPS and not the ADS-B GPS signal.