USB detect
To detect when USB for the interface MCU is connected, there is a circuit sensing the VBUS of USB connector J2.
nRF only mode
The nRF only mode disconnects the power supply of the interface MCU, the external memory, and the LEDs as well as disconnects the signal lines between the nRF5340 SoC and the interface MCU using analog switches.
Signal switches
On the nRF5340 PDK, there are multiple analog switches that are used to connect and disconnect signals based on different scenarios.