Where Photo Noise Comes From
Every camera sensor produces some random electrical signal alongside the real image. In bright daylight that signal is drowned out and photos look clean. In dim light the camera amplifies everything to compensate, and the random part gets amplified too. That is the colorful speckle you see in night shots, indoor photos, and anything taken at high ISO settings.
Small sensors suffer most, which is why phone photos show noise sooner than photos from large cameras. Older digital cameras, cheap webcams, and heavily brightened shots all share the same problem. Compression adds its own layer on top: JPEG saves space by discarding detail, and at aggressive settings it leaves faint blocky patterns that behave much like noise.
None of this means the photo is lost. The underlying image is still there beneath the speckle, and separating the two is precisely the job a trained denoising network does well.