I have some experience with indoor location tech, but not magnetic fields. If I had to guess (and I do) I'd suspect that the algorithm depends on consensus base

I have some experience with indoor location tech, but not magnetic fields. If I had to guess (and I do) I'd suspect that the algorithm depends on consensus based on traffic/usage. The map's gonna change regularly, but if enough people use the technology you can take the algorithm's guesses for everyone and start to build a confidence weight for your prediction, weed out anomalies, and update your understanding of the magnetic fingerprint of the space over time. I'd also assume it uses GPS to get your bootstrapped whenever possible, ie: sort out what entrance you came through, use pathfinding over time to get some limit the possible locations you might be in currently.