We should begin by saying that the point of all of this is not just to criticize one of the most repeated photographic myths out there – that's just a tool, because an incorrect understanding of the way that exposure and ISO work, as well as their roles during the shooting process, leads to a falloff in the quality of shots that photographers get.
So our goal is to show how, by rejecting this myth, you (the now-enlightened user) will be able to more fully utilize the capabilities that your camera has, as well as to improve the quality of your photographs.
Some say, "exactly like dragging exposure slider in Lightroom, ISO ... is just digital gain".
With a few exceptions, this is less than a half-truth.
Though it is often the case that at higher ISO settings digital "gain" is applied (or a converter, using a metadata tag, is instructed to apply it), analog gain still acts first, before "digital gain" (the better word would be "multiplication", not gain, as in electronics "gain" most often pertains to analog domain).
The giveaway for multiplication is deep and regular gaps in the raw histogram that become wider with each increase of ISO setting.
Fast Raw Viewer is the first and the only dedicated tool specifically designed and developed for extremely fast display, visual and technical analysis, basic corrections, sorting and setting aside or directly transferring for further processing of RAW images.