Small pixels inside image sensors do okay in bright daylight especially if the resolution is high enough, but they do poorly in low light or at night. This is because, light is the most important ingredient in a photo so if there’s no light, you’ll get no photos.
A good way to combat this is to make the pixels bigger in size, but then you’ll have to remember that smartphone camera sensor sizes are small. Some companies also like to keep the small pixels as they can fit more onto a slightly larger sensor and they’re good for marketing too. 48MP sounds way cooler than 12MP
So the other way to solve this small pixel problem is to combine several small pixels together to act as one. The most common method in use is joining 4 pixels together to act as one super pixel (pixel binning). This way the small pixels can catch more light and give out better shots but there is a catch. Pixel binning effectively reduces the effective resolution of the camera to ¼ of its original resolution.
Four pixels are combined together to form one super pixel (Credits: Gadgetstop)
In pixel binning, a 64MP camera may actually have 64 million pixels on the image sensor but it will output (64/4=) 16MP photos. A 48MP sensor can be binned to produce 12MP shots. This binning is actually done in the sensor by the companies that produce these cameras and not the phone makers themselves.
Many midrange SoCs cannot handle resolutions over 25MP so the camera sensor bins the image down. This binned images, for example a 12MP binned photo however contains a lot more details than a normal 12MP would because a lot more pixels helped to capture it. The camera sensor then hands over this 12MP binned image to the ISP in RAW format. The ISP then let’s the photo software on the OS edit the photo and give out the final image.
Despite being binned, these cameras are still capable of taking higher resolution photos through a manual mode where the user is allowed to tinker with advanced camera settings like shutter speed, ISO, focus etc. The sensor will still give binned photos to the ISP. The ISP will then upscale these photos to a higher resolution, maybe not as much as the 48 or 64MP but it could go pretty high.