As I understand, the feedback is serial input - parallel output. The drain voltage of the NMOS on the first stage is also the gate voltage of the upper PMOS of the second stage - therefore it determines the current in the second stage (current is a function of Vgs).
Now, The current in the second stage determines the voltage drop across the resistor, whic is the Vsource of the NMOS in the first stage. The current in the first stage depends on Vgs of the NMOS, so now the drain voltage of the NMOS changes accordingly - and this is your feedback.