Gradle was chosen to build Fabric mods. This document will dive into the details of the pipeline steps, and the tools used.
Yarn is an open-source repository that contains all the mappings used to turn Minecraft obfuscated names into the useful names Fabric mod developers use.
For more information on how to contribute mappings take a look here: Source
Enigma is a tool to deobfuscate Java applications, like Minecraft. Fabric uses a fork of Enigma that has been heavily modified to fix lots of bug and work more effectively with Yarn files.
The code is not for the faint of heart but can be seen here Source