It is not clear what exactly the question is here. So I guess:
You are missing ARDUINO 1.8.8 2018.12.07? What is wrong with the newer 1.8.19 2021.12.20 Version?
I assume that the microcontroller from microchip is the ATmega32u4,
which is well documented at Microchip and you can also download a library for USB there.
ARD MICRO WOH Arduino Micro, ATmega32u4, microUSB, without header
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For more technical details and starting guides, check out the Arduino website