Just read an advertisement for a Nikon D500. The ad says "native ISO 51200, Extend to ISO 1640000 for working in dark and difficult lighting conditions" ... this is for a used camera where the owner should know better. And here's the marketing blurb directly from Nikon: "With a native ISO range of 100 - 51,200, expandable to Lo 1 and Hi 5 (50 – 1,640,000 equivalent), the versatile D500 is not intimidated by difficult low-light conditions, down to -4 EV". The goal, of course, is to better the competition and claim the lowest EV.
These extreme specs by the manufacturer are absolutely ridiculous. I know, I have owned several Nikon D500. While the D500 is an excellent camera and certainly is amazing in low-light, there is absolutely no way that any D500 photo can be considered "quality" at ISO 51,200. Once you reach ISO 3200, the noise and artifacts from high ISO show up. Those that promote using post-processing for every shot will point out that noise can be eliminated in post-processing. That's not entirely true. You can eliminate some noise, but not all. You can't rebuild an image from data that isn't there, you can extrapolate and make it look more acceptable, but it is not contributing much to image quality.
In the film days, ISO was a term used to indicate film sensitivity to light. There were a number of ISO films you could buy: ISO 100, ISO 200, ISO 400, ISO 800, ISO 1200, ISO 3200. The most common are ISO 100 and ISO 400. The higher the sensitivity, the greater the cost. Actually, higher cost two ways: the price was higher and the quality was lower. Increased light sensitivy, or higher ISO, means the size of the silver particles in the film are larger ... that's where the description as more "grainy". That's the effect of higher ISO, larger and more grains of silver.
The concept for digital cameras is identical. ISO describes sensitivity to light. There isn't any film or silver to adjust,though. The adjustment is to the sensitivity of the sensor. While the concept is like film, the application is more like a CB radio where you have a "gain" button to strengthen (or weaken) a radio frequency. That gain may get you a stronger signal, but it will cost you in quality of the sound. In the digital camera, a gain is higher sensitivity described by the terms higher ISO. The cost in the camera world is the same as the film days: grain. We describe that "grain" exactly the same as the CB radio guys: it's noise.
The artifacts created by high ISO can be fixed, to some degree.
I have a Nikon D4S that is rated 100-25,600 natively. There is also a Low ISO setting of 50, and high ISO settings of 51,200-409,600. Dumb, dumb, dumb. Here's what Photography Life magazine says: "Anything above 25600 looks like trash due to heavy loss of colors, dynamic range and noise + artifacts.". On ISO 409,600 the reviewer says: "And I cannot understand why Nikon even made ISO 409600 available – it is completely unusable and there is nothing you can do in post to clean it up.". Here's a larger shot of the ISO 409,600 test photo. The only practical use of that photo is to show how completely ridiculous Nikon is even claiming that high an ISO range.
Doesn't it make more sense to shoot at the lowest possible ISO and get cleaner, crisper, and higher quality images?