Welcome to the forum! I’ll give you a partial response.
this link will help clarify that the 48mp camera is a quad-bayer sensor which indeed always going to give 12mp output. That is there are 48million sensors but this information gets converted into 12 m pixels in output. Edit: Beat to it!
On the Fairphone website, the “Cameras” section gives a better indication about digital zoom, looks like it has 8x zoom. For an explanation of the camera I would probably see if any tech reviews cover it. Of course you can install other camera apps on the phone, but I think few (definitely not open camera) are able to take advantage of the quad bayer sensor, so image quality is a bit worse there.