Look a little closer
A GPS satellite does not look down, recognize you and report your coordinates. Your receiver compares the travel times of signals from several satellites, estimates its distance from each one and solves for the point where those distances agree.
Each satellite broadcasts its orbital information and a precisely timed message. Radio travels at the speed of light, so multiplying signal travel time by that speed produces a measured range, more precisely called a pseudorange.
One known distance places the receiver somewhere on a huge sphere around a satellite. Intersecting a second sphere narrows the possibilities to a circle, and a third distance reduces them much further. This distance-based method is trilateration.
A phone's clock is not as accurate as the atomic clocks carried by GPS satellites. The receiver must solve for three spatial coordinates plus its own clock offset, so positioning normally requires signals from at least four satellites. Additional satellites can improve the solution.
The atmosphere changes signal propagation slightly, while buildings and terrain can block or reflect radio waves. Satellite orbit and clock errors also require correction. Models and correction data reduce these effects, but urban canyons can still make a position wander.
A map's blue dot may combine GPS with Wi-Fi, cellular towers and motion sensors. The foundation, however, remains a timing problem: convert several extremely precise radio travel times into distances and find the location consistent with all of them.
