Being based on V-USB which is open-source hardware/software, here is a description on how the uMIDI2CV module is built.
The module is based on an ATtiny85 MCU programmed with V-USB firmware that makes it able to handle USB communication without any USB controller.
The V-USB library can be found here:
The schematics is very simple
The ATtiny is programmed with the Micronucleus bootloader and is firmware upgradeable in the field.
You can built it yourself but the uMIDI2CV module is available prebuilt: