In Depth Architecture documentation

Hi guys,

I’m wondering if there is a UML or a doxygen-like documentation to get the global architecture of the source code.
The .sln is huge to load in msvc, and visual studio refuses to generate a graph from it. (either it cannot parse C++, or the graph of included files is so big that my pc becomes unstable)

I got this https://docs.unrealengine.com/latest/INT/Programming/index.html
but there is no explanation of the core structure : rendering loop, garbage collector internals, low-level network architecture, etc…
The aim is more to understand how the engine works than getting practical instructions to use it.

I think UE4 is an enormous C++ project from which I should learn. I begin to understand how preprocessor is generally used, but how inheritances are structured, of how the buildbot have been made remains unclear to me.