Comprehensive Unreal Engine Source Build User Guide - Where is it?

Many of us, myself included, want to build and package games while being unfamiliar with the intricacies of IDEs. I have read all the documentation Epic provides for working with a source-built Engine version. Yet I find myself really not knowing what is going on most of the time with my engine. For example:

  1. Why is my .git folder 82.27 GB? that seems excessive.
  2. Why is my entire engine folder (including .git) 386.64 GB? There has to be something wrong here. Its huge.
  3. How come when I change just a few parts of the engine source the IDE insists on building the entire engine again from scratch?
  4. What’s the proper way to install engine and project plugins from the marketplace when I can’t download them directly to the project or engine?

There’s probably many more questions I have I can’t think of right now. If somebody has written a guide for this I would love to see it!