The 7G package is the binary version of the Unreal Engine 4. It is downloaded and installed by the Launcher. Most people can make their game projects with this version.
The source code version of the Unreal Engine 4 is all the code that makes the Unreal Engine 4, and it is downloaded from GitHub. Users can modify the code to make changes to the Unreal Engine. These changes can give users extra control over the game they want to make.
I hope that explains it better!
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