please explain internal operation of network analyzer

I am trying to understand how our vector network analyzer is able to measure incident and reflected waves at a port.

I have read about directional couplers, but I am still not clear on how it can be used to calculate S11 for example.

A fraction of the incident wave power on the "input" port gets shunted to the "coupled" port. The voltage on the "through" port is the combination of the voltage that went through the coupler and the reflected voltage from the DUT. This voltage is not supposed to get into the "coupled" port. I am not sure how the value of the reflected voltage is found.

Are there two RF measurement systems, one that measures the "coupled" port signal and one that measures the "through" port signal.

Internal block diagram of VNA is given in Microwave Engineering by Pozar. But enough explanation is given in the book. I am attaching the block diagram. If you can figure out, explain to everybody.

If you look at the S-parameters matrix of an ideal coupler, you'll understand...