Once you eanble the external synchronization, it will configure external devices accordingly.
You can take a look at the source code in ~/nvidia/nvidia_sdk/DRIVE_Software_10.0_HW_Linux_OS_E3550/DRIVEOS/drive-t186ref-linux/samples/ directory for details.
The frame sync signal is from Xavier SoC generating a PWM signal on a GPIO pin that is connected to all deserializers. This common PWM signal is then forwarded to all 16 GMSL cameras to achieve frame synchronization. But it’s not configurable.