Ok so if you take Rf which is connected to Vo let's say. Take the voltage called Vo, make it like just a voltage dependent voltage source, one end connect to ground. Take the other end where Rf is connected along with the other resistor in series with that.
Now what you have is a voltage source with a resistor now shunted/parallel with Vs & Rin or Rs I can't tell the designator. If you do a Norton conversion it may be more obvious
Contrast, you have a resistor is series with the source, which forms negative feedback. That caused a voltage in series with the source Vi.
Voltage sources have to be connected in series (never shunt) and current sources have to connected in shunt, never series, So that means that the output voltage feedback forms a current that is in shunt with the input.
A simple inverting OP amps is the same essentially as this ckt.
I hope that helps.
