The Portenta family incorporates two 80 pin high-density connectors on the bottom of the board. This ensures scalability for a wide range of applications, offering more features and increasing connectivity.
In order to access the pins located in the high-density connectors easily during prototyping Arduino has developed the Portenta Breakout board. But if you'd rather implement your project directly you will need to know the connector type and part number:
The part number of the connector is DF40C-80DP-0.4V(51)
This will let you attach your own shields and connect components directly to the Portenta board.