The exposure triangle is the corner stone of still photography and differently, but no less importantly in videography.
If you get this knocked, then all the other stuff comes as needed, but if you cannot control these three in tandem, then the rest falls apart really quickly.
The basic concepts for those still learning are;
The three settings, Aperture, Shutter Speed and ISO all have dual roles.
They work together (or against each other) to get exposure right for the shooters needs, and contribute something creative into the mix, more or less. We will look at these with two criteria in mind. The first is creative control and creative potential, the second is damage mitigation or in other words, how the setting impacts quality if mis-used or improves it if used correctly.
Aperture gets a 5 for creative control and a 1 for damage mitigation.
Controlling the Aperture controls depth of field, which is for most photographers their first creative consideration. The landscape specialist uses deep depth, the portraitist shallow, but regardless, an uncontrolled aperture setting can force seemingly random and contrary results in your imaging. As for damage mitigation, if one of the other two settings is needed more for creative control, then the Aperture may be used for exposure balance.
Shallow depth of field, the main tool of the portraitist. Really brings out the character of the subject!.
Shutter Speed gets a 3 for creatively and 3 for damage mitigation.
Controlling the shutter usually takes the form of making sure it is fast enough to avoid motion blur, or conversely, to convey blurring deliberately in an often exaggerated form. In the past, film choices with fixed ISO’s and limited in workable range, forced an acceptance of some blurring in images, but as ISO tolerances become higher, perfectly still in almost any light is becoming more achievable.
ISO gets a 1 for creativity, and a 5 for damage mitigation.
ISO is the safety valve of the three. It determines the potential technical quality of an image and the sensitivity of the film or sensor to light, with the two extremes being directly opposed. If Aperture or Shutter speed are the primary creative controls, then ISO is often left holding the bag. ISO has always tended to be the limiting factor in the math that is photography. If you are using film, then the ISO is set, so film choice is a really big one to make, but in digital, ISO is flexible making life a lot easier. Very occassionally, a shooter will allow film grain or digital noise to become part of their creative process, more so with film or even video, but with so many processing options available now, noise is a poor substitute for a true film grain look.
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My standard working technique from the dawn of time (seems like) is to use Aperture Priority, or Manual with Aperture as the primary setting. This is because more often than not, I am using depth of field as my main consideration, or even if Shutter Speed is more important in a perticular image, the Aperture setting is still controlled to some extent.
With Micro Four Thirds, ISO is the enemy, much as it has always been in photography, but many larger sensor cameras are starting to make it effectively an irrelevance. This means that using Aperture priority usually results in just settting the Aperture wide open as f2.8 on MFT which is f5.6 (at the same effective lens length) on a full frame. A good, safe working aperture with decent light gathering power.
Sometimes the balance between ISO, Shutter speed and Aperture can be vexing, but mastering it allows you full creative control.
I also tend to control the ISO, by either limiting its range in auto ISO or set it speficically to what works in a specific situation.
Very occasionally, I get caught with a slower shutter speed than I would like as I have allowed the camera to use it as its safety valve.
The solution, is to set the Shutter to what ever I feel is the minimum a lens would need, usually doubling the lens focal length at least (in MFT), or 1/250th is a good all rounder.
If the Aperture is effectively irrelevant, as wide open will never cause undue harm, and ISO is less of a problem now with better processing options, with the Shutter is locked in, I will never have a blurry image at least. This may not work with fast primes as f1.8, even in MFT format may be too shallow in focus depth to work, but with an f2.8 zoom, I think it would work well.
Of course the biggest issue is getting into habits that are contrary to my SOP for the last two decades or more.