Mesh Datastructure C++

I’m currently thinking about using Netgen/NGSolve on the native side for an extension project. I digged into the source code but couldn’t figure out how the mesh database actually works and how everything is connected to each other. Is there any additional material available or can someone give me a brief overview over the general idea? I’m used to C++, Python and Pybind11, so don’t worry about that part. Thanks!