So there are three USB modes:
1. Power off and connect USB - STM32 Bootloader
This one will need drivers (first one is recommended)
https://impulserc.blob.core.windows.net/utilities/ImpulseRC_Driver_Fixer.exe
http://open-txu.org/home/undergraduate-courses/fund-of-opentx/using-zadig/
DFU Flashes both the bootloader and the firmware.