The range of light of the scene is broken up into Zones. Zone 0 is black and devoid of any detail. Zone X is pure white also devoid of any details. In the middle is Zone V or middle gray. This is important to remember, as every properly working light meter will measure brightness and indicate an exposure that will place that brightness in Zone V.

For This Discussion Spot Metering Is Used

The basic or starting foundation for exposure is determined thus:
Basic f/stop = 1/square root of the ISO

Basic Shutter Speed = 1/number of foot candles

This will place the part of the scene metered in Zone V. This is an inherent characteristic of light meters. Changing either shutter speed or f/stop will place that part of the scene in a different Zone, darker with less exposure and brighter with more.

So, if we are shooting a scene using ISO 125. The basic f/stop is f/11 (11^2=122, which is close enough)
Spot metering a part of the scene indicates that the brightness is 250 ft-candles. For this example, the basic shutter speed and f/stop combination would be 1/250@f/11.

If we were to change the shutter speed to 1/500 that metered part of the scene would now be placed in Zone IV. Similarly if we were to change the f/stop to f/16, again the metered part of the scene would then be placed in Zone IV.

With black and white film, the idea is to spot meter and place the darkest part of the scene in Zone III and see where the brightest part of the scene falls. This will determine the amount of time needed to develop the film to achieve the necessary tonal range of the negative to match the paper on which the print is to be made.

With digital, the situation is just the opposite. After determining the individual camera's capability to capture highlights and still maintain details in those highlights, one can spot meter the brightest part of the scene and increase the exposure to place that part of the scene in Zone VIII or IX, thus maintaining some details in the brightest part of the image.

I posted three articles exploring the basics of exposure and why a triangle is not the optimum way to think about the relationship of shutter speed and f/stop combined with ISO. You can find them here.

Exposure and How It Works - A Beginner's Guide
http://www.uglyhedgehog.com/t-484554-1.html

More Exposure and How It Works - A Beginner's Guide
http://www.uglyhedgehog.com/t-484826-1.html

Finishing Up Exposure and How It Works - A Beginner's Guide
http://www.uglyhedgehog.com/t-485845-1.html

For further reference:
"A Memorial Volume Containing an Account of the Photographic Researches of Ferdinand Hurter & Vero C. Driffield : being a reprint of their published papers, together with a history of their early work and a bibliography of later work on the same subject" - Authored by the Royal Photographic Society of Great Britain
,
"The Negative" - Ansel Adams

Although these references discuss film sensitivity and processing, there is some correlation to digital. "The Negative" discusses the foundation of exposure.
--Bob

IMO the foundation would be that you learn how each element (shutter speed, aperture, ISO) affects the image - not just light and dark, but depth of field, motion and noise.

You learn that many combinations of those three elements give you exactly the same exposure* and you learn what happens when you only change one of the elements (exposure compensation). You also learn how your camera measures light and dark and why it suggests, or selects, certain settings.

Someone with a foundation of exposure knowledge would not ask "What settings should I used for xxx" unless it is a relatively unusual occasion (eclipse, northern lights, night sports) and would never wonder why their photos were blurry (if the problem was motion, either by the photographer or the subject). There would be no need to ask how to get a background to go out of focus, or the opposite - why the background is not sharp.

*learn what a "stop" is:
https://photographylife.com/what-are-exposure-stops-in-photography

btw, I still find it odd when someone says, "Your shutter speed is too slow to capture a bird in flight; raise your ISO." To me that's like saying, If your home is too hot with the furnace set at 78, open the window

If shutter speed is too slow for a subject, you increase the shutter speed and then work out the rest of the exposure (or let the camera do it if in shutter priority). Maybe 'cause I was raised on film where the was no "changing ISO" (yeah OK, except for push processing)?

On the thread you included, a number of UHH members debated the value of buying the 4th edition of Peterson’s book since they already owned one or more previous editions. I think if you own a previous edition, you don’t need to go back over essentially the same ground by buying the 4th. On the other hand, if the first copy you buy is the 4th edition, you are choosing an excellent reference with a good grounding on some key basics. As other UHH members point out, Peterson does not attempt to fully define or cover every aspect of the diverse subject of exposure. There is more to it than just the triangle, as serious photographers learn over time. But it is helpful to start with those basics.