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, external memory, and LEDs of the interface MCU. It also disconnects the signal lines between the nRF52840SoC and the interface MCU using analog switches.
Signal switches
On the nRF52840 DK, there are multiple analog switches that are used to connect and disconnect signals based on different scenarios.