SPI communication

I am using a STREAM FPGA board connected to an RF board.

Up until now, Quartus has been able to output frequencies to the NCO IP.

Next, I want to send the output frequency to the transmitter of the RF board and confirm it through the spectrum analyzer. I do not know how to connect the two boards (RF, STREAM).

I do not understand why the downloaded user guide uses SPI techniques.

I would like to get good advice or related guidance for me.