These directories provide an API to open and close a u-blox device, i.e. a u-blox chip or module. It is intended to be used in conjunction with the common/network API; see the README.md) there for usage information.
You will notice that there are _stub.c files in the src directory; if you are only interested in, say, cellular, and want to leave out short-range/GNSS functionality, you can simply replace, for instance, u_device_private_short_range.c with u_device_private_short_range_stub.c, etc. in your build metadata and your linker should then drop the unwanted things from your build. You will need to do the same for the GNSS and Wi-Fi/BLE (i.e. short-range) components in common/network/src.