Basic Basics
At 90 mph:
Every image wants the same amount of light.
-- The shutter regulates that light by changing the TIME that the sensor (or, in the old days, the film) receives light. More time, more light.
-- The diaphragm regulates that light by changing the aperture, or the SIZE of the opening. Bigger opening (small f number) the more light.
-- The ISO is a measure of the SENSITIVITY of the sensor (or, in the old days, film "speed.") The higher the ISO number, the more sensitive ("faster) the sensor.
Grossly, all are reciprocal, that is you can trade one for the other. Each, however, has a secondary effect, that that is why you use one or the other for effect.
-- Fast shutter "stops" motion (think sports), slow shutter blurs the motion. (think silky waterfall picture)
-- Wide aperture creates shallow depth of field, only a small part of the image is in focus. (portrait, perhaps, so that the background blurs) Small aperture creates sharper image and greater depth of field. (landscape)
-- Higher ISO is "noisier" and creates a grainer image. But grain may be better that a blurry image because you can't hold the camera steady, long enough.
Fairly easy to understand, takes a lifetime to develop the instincts to know when to use which. Hope this helps.
https://photographylife.com/iso-shutter-speed-and-aperture-for-beginners