I did some quick experiments with Phyphox on my FP3 to find the location of the magnetic field sensor. I did this in part because I like pretending I’m smart and using my phone as a stud detector, but that only really works if you know where the sensor is in the first place
I essentially laid the smartphone flat on a table, overhanging upside down so I could read out the measurements and simultaneously move some magnetic objects over the back surface. I got the strongest magnetic signal when the magnetic object was close to the lower left tip of the ?Wi-Fi antenna (not sure if this is what it is, I recall seeing it somewhere in the forum)?. This antenna is the square shaped metal feature to the direct right of the fingerprint sensor in this image-- photocred to ifixit --..
So, based on only this, I’d say that the magnetic sensor is in the main module, in close vicinity to the wifi chip you can see in the teardown. However, in the teardown they also mention that the ambient sensor is in the top module. So it’s possible that the magnetic sensor uses the antenna as amplfication and is in a different place?