How research game internals?

I want know how i can research game internals?
1. What packets game send to server and receive? I can sniff packets, but this only bytes, no real information like variables names or other.
2. Many cheats use game internal structures, but how authors research this internal data types? Debugging? Decompiling? Source code?
3. Some tutorial links about research game internals?
4. Reverse-engineering for take some internal structs, classes?

If you know for example the pointer of a player class you can use ReClass/Structbuild(most use tool for reconstructing classes) to monitor values in realtime and map them to what you do ingame(by giving them names)
For function names usually it's a matter of strings referenced in ollydbg.

Or you could try sticking a knife into the program and examine it's entrails. That's a good way to research it's internals :P

Usually you will never get any variable names unless you are very lucky (e.g. debug build + PDB leak). Packet format is documented by observing the variables used to construct the packet in the first place (trace back from send/sendto/WSASend/WSASendTo and observe how the buffer is manipulated). There are many ways to discern game internals and all the choices you have listed are certainly viable (provided you have the source code for the last option).

For polymorphic classes, learning how RTTI is implemented by certain compilers will help you reconstruct classes.

You also need to know the codding

The answer is the same for everything... reverse engineering was applied.

Those private wow servers are not identical to the real server. They don't have access to the real server. The people who build private servers study the response the server sends to some type of request, then they implement that in their own server.