Very rarely have I seen anyone really learn anything from just looking at other people's source code without some sort of explanation attached to the code. It doesn't do you much good to see what's there if you don't know why it's there.
You should probably look into some engine design books. I personally learned a lot from Game Physics Engine Development. It actually went into why did things the way it did, alternative ways to do things, and the cost/benefit of doing it different ways.
Read a book. It will be much more helpful. I tried learning from other 3D engines. It's either too complicated to see the whole picture or too basic (does not contain what you want to know). A good book is 3D Game Engine Architecture. I am reading it right now. It explains how to write a 3D Engine and uses WildMagic engine as an example engine.