What voltage or node are the BJT substrates connected to in a bipolar process opamp IC?
Substrate is normally connected to the most negative voltage in the circuit (Vee in the case of the 741) since it is p-type. Since the emitter and base are built on top of the collector diffusion in a vertical NPN, the collector-substrate junction characteristics are of interest.
Lateral PNP transistors as you'd find in the 741 are going to be different, I assume one would have a particular model for such transistors built in the process in question and not use the standard PNP model.