If one connects the phases of a wye configured 3 phase supply to the phases of a wye configured load, and also connects the common or neutral nodes together, then, the voltage from one neutral node to the other will ideally be \$0\$. Being connected by a wire, they are, when considered ideal, the same node.
If the phases all have the same sinusoidal voltages, are all \$120^\circ\$ apart, and the loads are identical, them no current will flow through the neutral wire.
When the above three conditions are met, the circuit behavior would be unchanged by removing the neutral wire, or replacing the neutral wire by a path through the earth.
If the three-phase voltages are square waves, is there a neutral voltage in a balanced wye-wye configuration?
If a wye connected power source with square wave voltages between each phase and neutral is connected to a balanced wye-load with the neutral of the supply not connected to the "neutral" of the load, then the two "neutrals will NOT maintain equal potentials. Rather, (assuming purely resistive loads) a square wave of frequency 3 times that of the power supply will appear between the two "neutrals".
simulate this circuit – Schematic created using CircuitLab