I just set black and white point in the histogram, which was perhaps not completely identical for both images. Thus the appearance of a slight difference in saturation. That was all for both images. So I did not change the pixel values – in contrast to the procedures you mention (brightness, exposure, gamma …)!
Concerning the issue with ISO I want to quote from the book by Tooms (2016): “Colour reproduction in electronic imaging systems”: On page 407 he writes: “The image sensor will have an opto-electro-conversion factor (OECF), which is normally linear and will output its signal to variable gain amplifiers which are adjusted for minimum gain, consistent with providing a standard level signal after the desired adjustments to the iris and shutter speed exposure settings. The gain adjustment is usually calibrated in terms of the ISO rating, which in turn is related to the ASA film sensitivity or speed rating familiar to traditional photographers.”
This confirms that the signal is just multiplied by a gain factor, which in turn does not change the SNR.
I also found a webpage, confirming what I just wrote. Scroll down to the part about “Common ISO Myths and Misconceptions” and read the paragraph “Is ISO “Sensor Sensitivity”?” The fundamental difference to the ISO/ASA of a photographic film is, that a film with high ISO does indeed have a higher sensitivity. If you raise the ISO setting of your camera, it does not increase the sensitivity of the detector.
However, one never knows what the firmware in the camera does in detail, if one raises the ISO setting. Thus it may well be, that due to some clever algorithm the image with high ISO setting does look somewhat better than what you get with a lower ISO setting and some image processing with the software of your choice.
Hermann-Josef