Which Unity version should I use ?
Unity 2019.4.25f1
Where is the build script ?
In Assets/Editor/Build.cs. You can call functions ClientBuild() and ServerBuild() inside the Build static class with a command line.
CI/CD
The CI/CD specs are located in .github\workflows\main.yml.
Misc
The main code location is situated in Assets/Scripts. There are 3 solutions : tests, assembly-ubv, and Assembly-CSharp-Editor, which respectively manage unit tests, game code and editor custom inspectors.