Tangentially related, here is the opensource gps software used by a lot of phones:
https://gpsd.gitlab.io/gpsd/index.html
Development primarily lead by Eric Raymond:
https://gpsd.gitlab.io/gpsd/index.html
Development primarily lead by Eric Raymond:
Actually taking the raw waveforms from the ADCs and turning them into a (position, velocity, time) solution is an entirely different task.
I'd be remiss if I didn't mention RTKLIB, which is an open-source package to do the PVT solving, using a whole bunch of different algorithms. It ALSO happens to include a bunch of plumbing to stick the pieces together, and thus in some ways overlaps functions of gpsd, but its primary function is actually solving the navigation equations.
It doesn't do much beyond multiplexing/marshalling. The actual calculations are done in the receiver.