https://photographylife.com/what-is-iso-in-photography
When you double your ISO speed, you are doubling the brightness of the photo. So, a photo at ISO 400 will be twice brighter than ISO 200, which will be twice brighter than ISO 100. Optimally, you should always try to stick to the base ISO (lowest ISO possible) to get the highest image quality. However, it is not always possible to do so, especially when working in low-light conditions.
https://photographylife.com/exposure-value
https://photographylife.com/what-is-shutter-speed-in-photography
https://photographylife.com/what-is-aperture-in-photography
https://photographylife.com/f-stop
The f-stop, which is also known as the f-number, is the ratio of the lens focal length to the diameter of the entrance pupil. If you did not understand that, don’t worry, because there is a much easier explanation of it for beginners. In very simple language, f-stop is the number that your camera shows you when you change the size of the lens aperture.
https://photographylife.com/landscapes/what-are-histograms-a-photographers-guide
Apparently ISO 100 is the default for any material online.