Beginner question: ISO
I suppose you can limit the auto-ISO working band.
All sensors will have noise, but when the signal is strong the noise will be unnoticeable. In low light the signal will be weak, and the difference between the signal and noise smaller. So when you increase the signal you will be increasing the noise as well. This makes noise more noticeable. When light is very low, the difference between the signal and the noise can be very small. So when you amplify, the noise will be almost as strong as the signal, making the photograph unusable.
Think of the ISO setting as a volume control. I don't know if you ever got to use a cassette tape, but it works in a similar way. If the recording was good, you wouldn't hear the tape hiss (noise). But if the recording was to low, you'd need to increase the volume to hear. And doing this would also increase the tape hiss making it noticeable.