Purpose of STM32F103 on Cube (Pixhawk 2)

We intend to develop custom firmware for the Cube (Pixhawk 2).

We have reviewed related documentation and posts such as:
https://docs.px4.io/en/flight_controller/pixhawk-2.html


https://dev.px4.io/en/concept/architecture.html

Despite reviewing the posts above, we have not been able to understand the intended purpose of the STM32F103 within the context of the PX4 cube system. Furthermore, how does the STM32F103 and STM32F427 communicate? Additionally, which firmware and how is it uploaded to the STM32F103?