In brief, here’s how it works: Each satellite in the GPS constellation, which is maintained and updated by the US Space Force, carries with it an extremely accurate atomic clock. These satellites beam down radio signals that carry two key pieces of information: the satellite’s current location (i.e., latitude and longitude) and its onboard clock time.
These signals move at the speed of light to reach GPS receivers on the ground. Based on that constant rate of motion, a receiver is able to tell how far away a satellite is by calculating the difference between the satellite clock time and the local clock time whenever it hears a signal. If there are at least four satellites overhead (which the current constellation of 24 satellites, plus a few extra, all but guarantees), the receiver can use trilateration to figure out its location on earth. Some 95% of the time, the system is accurate within seven meters.
https://www.bloomberg.com/news/newsletters/2023-09-28/maplab-the-secret-history-of-gps