This is just a very basic overview of the sources.
At a later time, more specific information may be added to
http://www.orbiterwiki.org/wiki/FlyByWireEGCS

The FlyByWire source code consists of three parts:

I. C++\FlyByWire\*
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A simple Visual C++ DLL, needed as an interface between Orbiter and the FBW Core DLL (which is written in Delphi 7 and thus not able to directly communicate with Orbiter).
This is mostly based on GPL'ed code from Artlav.

II. Delphi\FlyByWireCore.dpr
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Delphi 7 Project. This is the heart of FlyByWire.

III. Delphi\GuiDevTool\GuiDevTool.dpr
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A little helper project. Basically, I used it to create the layout of the GUIs for different controls in the setup dialog.
The program generates simple source code for creating this GUI on the fly, which I copied and pasted into the FlyByWireCore project. Probably not the most elegant solution, but it got the job done.