48 megapixels doesn’t necessarily mean 48 megapixels. Android phones for a while now have used a technique called pixel binning. This is a technique whereby the camera sensor’s pixels are programmed to work together as if they were one pixel and thus get the added benefits of larger pixels (ie: better light gathering, less noise). Typically it’s four pixels grouped together to form a single pixel on outpost, thus the resulting output from a 48 mp sensor using pixel binning would be a 12 mp image. This allows the camera to be somewhat flexible. In ideal light it can shoot up to the native 48 mp, or it can use digital zoom and output 12 mp cropped from the native 48 mp, or it can turn on pixel binning for better performance in low light.
The technology’s been around in the Android world for several years. Apple tends to adopt these technologies only when it feels they’ve matured enough for prime time so it’s likely they’re working on an implementation of this for iPhone.