MOSFET as a Capacitor in IC
Obviously the other plate is S (which generally is connected to the substrate) and D (which is connected in this case also to S) and the channel inbetween.
S and D both together and the channel form the second plate (you could also use only S but that would even increase the unwanted resistance)
In the center picture the second plate is represented by the the line between both resistors. So the two restsors in the center picture are two series resitors connected in parallel. You can replace those two resistors by a single series resistor (right picture). See modified picture below:



Note:
If this derivation seems somewhat sloppy the sloppy part is not between 2nd (center) and 3rd (right) picture but between the 1st (left) and 2nd (center) picture.
Without mentioning there is a transition from an actually distributed element situation (the channel resistance is distributed over the whole plate which is part of a capacitor) to a lumped element representation.