Trouble visualizing voltage-shunt mixing
Visualise In- as a virtual earth.
All currents into that node will sum to 0 (the input current of the opamp, which is infinite impedance) and the voltage at the node will also be 0 (because In+ is hardwired to 0, and open loop gain is infinite)
These two constraints define its operation (within its linear range).
Therefore mixing occurs through the summation of all input currents plus the output current, whose result is 0.
Therefore the output voltage is whatever is required to oppose the linear sum of the input currents through Rf.



simulate this circuit &ndash; Schematic created using CircuitLab
&quot;Shunt&quot; effectively means &quot;connected in parallel&quot; and hopefully you can see that the feedback path (to virtual ground) is in parallel with the load (to actual ground).
I hope you can see this circuit is both identical to yours, and identical in topology with ONLY your &quot;Voltage Shunt&quot; diagram.