ISO = International Standards Organization for light sensitivity to film and digital photography
I used Nikon Top 'flagship' FX D5 and DX D500 cameras to test ISO grain/noise qualities

D5 and D500 High ISO Test; blowing the D500 to 51200 ISO and the D5 to a whopping 103,000. Each photo has the ISO in the comment.
Cameras used the same lens, 24-120 F4 AF-S lens, mounted on a Gitzo tripod. Overcast, dull but even-consistent lighted day.

First two reference photos were edited as how the full frame highest ISO photo, resized and edited for the web use, just for reference. Editing on first two are noise cleaned.

I show the D5 first - then the D500 photos. The photos were set to 100%, and a center cropped and used, 1800 wide sized.

Extra, I also made a Dfine edit on the 3 higher iso images to show how it works, just for information.