How to implement UART over USB on nRF52840-DK

Hi I found one example project named "serial" that implemented UART over USB:

There are 2 USBs on nRF52840-DK board, USB J2 and J3.

J2 goes to ARM processor, J3 goes to Nordic chip nRF52840.

This project implement UART over J2.

My company will not put ARM processor on out product, only Nordic chip nRF52840, which means I need to implement UART over J3.

Is there any code that implements UART over J3 directly?

PS: there's another example project named UART, what's the difference of this one compared to "serial"?