You git clone it from github, then edit and ./gradlew build from there. That image is just downloads. Mod is the mod itself. Changelog is that changelog version. Defaultconfigpack is just the default config pack as of that version, Dev is the development linking library jar (for developers making mods on top of gt6, and Sources is the development sources library jar (again for developers making mods on top of gt6). For modifying GT6 itself you git clone it from github as it is a fully self-contained standalone ready-to-build project, very frictionless compared to basically any other mod anywhere. 
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