A question on Thevenin and output impedance
The green circled one represents the output impedance of the whole circuit. the red one is a component of it, but not all.

In theory you could measure the Thevenin resistance by shorting the supply, but if this were a real supply, it would likely protect itself (short circuit protection), and you wouldn't get the correct answer. Even if it didn't protect itself, if it has any nonlinear or active components (opamps, diodes, transistors etc.), the behavior of these devices when shorted might be significantly different (in a non-linear way) from the lightly loaded behavior and so you wouldn't get a good result.

Best approach is to put a light load (say full scale load) on the supply and measure the change in output voltage -- now Rth is deltaVOUT/ILOAD