Answer:
The GPS receiver, receives time signals from satellites overhead.
The difference in time, for the signal to reach the receiver on earth, will be different from each satellite and can be measured.
The time differences are applied to a mathematical formula and give the position on earth, in three dimensions (height as well).
Additional information from the satellites will identify each one.
An almanac built into the receiver, will determine where each satellite is in the sky. (or where they should be)
This positional information is used in the calculation and the time difference signals applied to it.
The system is passive. The GPS receiver does not transmit or communicate in any way with the satellites. It listens only.
The atomic clocks onboard each satellite are monitored from earth and kept in synchronization.