The plotter and situation view orientation can be changed from north up, course up, or heading up.
When the orientation selection is heading up, the Cortex automatically selects the best sources of heading data. In this order, an external sensor (if available). If none is available the heading source uses COG, when SOG is >2 knots, then Internal Heading (if it has been calibrated). You can tap on the heading indicator at the top of the view and Cortex will tell you the source it has selected.
The heading up with fallback can also be displayed in your instruments view. On the heading sensor data view tap the ... and select Heading with fallback.