I have been shooting Nikon digital cameras since 2009. When I started looking at digital cameras, I was going to buy Nikon or Canon. I went with Nikon because of Auto ISO, which Canon did not have at the time. I shoot Auto ISO most of the time. The only time I typically turn it off is if I am doing night photography or flash photography.
With Auto ISO, I can select the lowest and highest ISO the camera chooses. I have my Nikon D850 select between 64 to 25,000.
With older model Nikon cameras, the Auto ISO works by selecting a minimum shutter speed. In the menu, you choose the slowest shutter speed the camera would shoot before increasing the ISO. The problem is with a variable focal length lens, you can shoot a slower shutter the more you zoom out. When I started shooting, I would put my camera at 1/100 for the minimum shutter speed. I had a Nikon D80 and shot mostly with a 17-55 F/2.8. This lens did not have VR, so that value usually worked for me.
Around 2012, Nikon added the option to set the shutter based on the focal length. With my Nikon D850, I have my Auto ISO set by the focal length. This way, I can shoot slower shutter speeds at lower focal lengths.
Auto ISO lets me shoot faster. I can be taking photos inside at a high ISO and then run outside, and then the camera will select a lower ISO.
It is easy to turn off Auto ISO. On a lower-end Nikon camera, you go into the menu, and on a higher-end camera, you can turn it off by holding down a button and flicking the dial.
I recommend using Auto ISO if you are shooting Nikon cameras.