I'm not really understanding what your question is here. As I understand it, variant
is about getting a desired exposure out of the camera based on native sensor technology (some handle signal amplification better than others). How the camera uses that technology will then determine which exposure technique will work best during and after the time of exposure, like ETTR, UniWB or just simple old fashioned PP with exposure correction and noise reduction, during capture.
I have noticed that the old mantra of shoot at the lowest possible ISO
doesn't always give you the best results. Different lighting circumstances will dictate which shooting technique will work best. Factors such as subject activity play an important role, specially if you are looking to implement a base ISO/boost exposure later
workflow. A lower ISO may require a slower shutter effectively creating too much blur on a moving subject. Raising the ISO negates the base ISO/boost exposure later
workflow but gets the desired effect in camera.
As for a good explanation of ISO invariance, I found this article; ISO Invariance Explained