How do you remember all the game features while developing a private server?

Hi,
I played CO long time ago, before ninjas and other new features were released. I am interested in learning how to develop a private server in my free time. My issue is not learning how to code since I'm a full time software engineer and I know well that some dedication and time will let me learn how to do so.
My main concern is, how do you remember all the features and details of the games if you left the game long time ago? Is it better to play the official game to remember everything, and learn about the new features? Is it better to join a private server instead? Is it better to download an almost-complete source code of a private server, and continue building on it?

Edit: One of the main reasons I asked this question is that, currently I'm not really interested in wasting time playing the game and leveling up to discover everything. I assume that there's a more efficient method for programmers to discover everything, and yes they have to play but not consistently just for the sake of developing a private server.

If you have any other advice feel free to write them down.

Well, I played for over 10 years and that was enough to remember a lot of things from the game. If you're willing to get a older version, you may just look for a <5072 leaked binary and play on this to get a retail gameplay, or join some servers to see what people do nowadays.
There are other boards that offer sources that try to act like retail... "canyon world conquer" may be a google query

I don't. But I'm not trying to mimic / emulate the original game exactly.

Played for a while, so I remember a lot, but otherwise, reversing leaked binaries and using the Wayback Machine to access the old CO website is good to get the actual features. After, depending on how accurate you want to be, you can just look at existing sources (while they tend to all diverge slightly from Retail).

Thank you all for your answers. I don't want to mimic the original game of course. I was just curious how private server developers remember the core features and quests of the game.