The GPS satellites all transmit on the same two frequencies- L1 & L2.
The L2 signals are encrypted – used by the military for more precise positioning.
The 1574.52 MHz L1 carriers are modulated with a 1.023MHz binary pseudo-random code (Coarse acquisition C/A code). These are strings of 1023 bits repeating every millisecond.
Each S/V has a different code.
The GPS receiver generates the same C/A codes – by lining them up it can pick out the very weak spread-spectrum signal out of the noise.
The receiver searches for the delay time that locks the signals together.
The signals from other S/Vs don’t correlate. CDMA also used by some cell-phones.