The brains of the board is the SAMD21G microcontroller, using the 32-bit ARM® Cortex®-M0+ processor (the same MCU is used by Arduino ZERO and Adafruit Feather M0 boards). It comes preloaded with firmware; normally there is no need to change it.
If you want to learn more about the firmware, install an update, or compile your own firmware from source, please visit RoverWing Firmware github repository.
The MCU communicates with the Feather board via I2C bus; it uses the slave address of 0x11 (or 17 in decimal form).