What you really need to understand the interaction of aperture, exposure time and ISO sensitivity is surprisingly little and can be found here. Explained in simple words with the ©exposure mixer from digital-photography.com.
All three together make up the exposure and determine the brightness of a photo. You can think of the interaction like a mixer, with three controls that each act like an amplifier for the image brightness.
The latitude in the aperture is limited by the lens and the camera. I have indicated this in the illustration by showing the top and bottom values in grey.
The exposure time has the most leeway, can be as short as 1 ⁄ 8000 s and as long as you like. Smartphones often do not expose longer than 1 ⁄ 4 s.
The ISO scale also varies in length depending on the camera. Upwards, top cameras allow a few more steps than the ISO 25600 shown; because of the continuous doubling, this gives very high values of ISO 102400 or even more.
In the picture, the slider for the aperture is set to 4. If you open it to 2.8, for example, i.e. move the slider up one stop, the picture becomes brighter.
But if you reduce simultaneously the exposure time by one stop to 1/250 s, you compensate for the wider aperture and the brightness of the picture remains unchanged. Instead of shortening the exposure time, you could also reduce the ISO value by one step from 200 to 100 - this would also keep the image brightness constant.
If you move one slider and want the image brightness to remain the same, you must also move another slider in the opposite direction. Or both others respectively less.
For any desired image brightness, there are a number of combinations that achieve the same result. Selecting one is the task of exposure control and the subject of another chapter.
When the subject becomes darker, the exposure must be more generous. To do this, all three sliders must move up in total if the image is to remain equally bright. And even if the image is allowed to get a little darker, you will have to increase the exposure in dim light to avoid getting black shots.
You will then hit limitations in low light:
Then the room for manoeuvre is limited, the only thing left to do is to increase the ISO sensitivity.