Well, there are several ways, but many phones are actively mapping locations all the time to some degree, but almost all the big boys have very ambitious mapping versions coming down the pike. Even some of VR stuff can be extended into quasi-mapping as well.

Beyond that there are several ways that the grid can echo-locate sort of....these towers need to be very close together (because penetration and small packets) so getting other readings, IR for example, magnetic, thermal etc could be very easily integrated. We're talking about a tower at every corner in a city minimum. These towers need to be very close together.

So most likely it will be a hybrid of a ton techs. Face recognition. IR, cell triangulation. Thermal maybe. Basically it's just a giant unregulated grid of information. But if you wanna go for a tin-foil deep dive I recommend looking up stuff about the relationship between the FBI/CIA/NSA and big telecoms.