Why the guide you need depends on where it fails
A camera fault has two possible homes: the device, or the software asking for it. The browser camera test separates them in ten seconds. If your camera works there, the hardware and driver are fine and you need the guide for whichever application is failing. If it fails there too, you need the guide for your operating system.
That distinction saves most of the time people lose on camera problems, because the two categories have almost nothing in common. An app that has selected the wrong device and a driver that has stopped responding produce identical symptoms and need completely different fixes.
Testing for a specific application
Each of these covers that app's own device settings, its permission model, and the failures peculiar to it.
If your camera fails in several applications at once, one of them is probably holding it. Microsoft Teams is the usual culprit because closing its window does not quit it. See camera in use by another application.
Testing on a specific device
Operating systems differ more than applications do. Windows splits camera permission across three switches, macOS grants it per application and needs a relaunch, ChromeOS has no drivers at all, and Linux depends on kernel modules and sandboxing.
Start with the test itself
Before reading any guide, run the test. The readout tells you what your camera is actually delivering, which is often the answer on its own.
Measuring rather than fixing
If your camera works but the picture is poor, the problem is usually measurable. The resolution test shows whether a camera sold as 1080p really sends 1080p, and the frame rate test demonstrates how much of your image quality is lighting rather than hardware.