Figue-1:
In Fig. 1 above, we can see four communication buses (Bus-2 does not exist). Among them, Bus-3, which is the Serial Wire Debug (SWD) bus, is dedicated to programming the MCU flash memory with code and data.
Bus-4, which appears as ttyHS1/UART2 on the MPU side and LPUART1/Serial2 on the MCU side, is dedicated to the Router Bridge for high-volume data exchange between the MPU and MCU.
This leaves Bus-5, which you can use for high-speed data exchange between the MPU and MCU. It is based on the SPI interface. To use this bus, you need to write a C++ sketch for the MCU side and a Python script for the MPU side to exchange data. You may find this thread @philippe86220 interesting.