System Workbench for STM32

I will try to be more detailed and precise.

Using STM32CubeMX, I have FreeRTOS and CDC running on a STM32F4Discovery Board.

FreeRTOS is running four Threads that flash the boards LEDs, each at a different frequency.

main.c calls MX_FREERTOS_Init(),

MX_FREERTOS_Init() calls StartDefaultTask(),

StartDefaultTask() calls MX_USB_DEVICE_Init().

I follow this program flow.

I know that CDC_Receive_FS() is being called to receive the data from the VCP, but I do not know how this is happening. Which function is calling this function or which or how is the data received interupt connected to this function?
I am stuck understanding how this function actually gets executed. It's been many years since I did any real work with C (Z80) and I am struggling here. Tere must be some simple piece that I missed. I have read all the USB Device Library manuals and stuff about CDC and VCP and USB from STM, and I don't get it, yet.