its a current amplifier and has a bandwidth of about Fsw/12.
It output will be zig-zag, but will have the 100Hz envelope on it, as its inputs are from the current and line voltage.........which are pulsating at 100Hz.
It gets compared to the oscillator via the PWM comparator.
Obviously, the Duty cycle will change slowly, and with again an envelope of 100Hz.
...The voltage amp, current amp, and multiplier , combine together to make the current follow the Bus voltage "humps".
so basically reverse engineer it..
-you know ifs pfc
-so you know the duty cycle will change with a 100Hz envelope
-so you know that the output of the current amplifier will basically be a 100Hz waveform (in steady load condition)