ISO

ISO uses the light from the surrounding area from your picture.
When you use flash you mess up the picture because they flash takes away the natural light, and adds a bright shine to your skin.
Over the course of the pictures the camera is taking the light from the picture and it makes them brighter.
I would use ISO over flash when I’m outside and there is a lot of beautiful natural light around you.
Have to be careful when you use a fast ISO because your camera might let in to much light.