Need help finding a tutorial, pdf, or some sort of Documentation

I want to get the understanding of how to put together a basic system of a game. For example, the game will first consist of creating a character (Which is saved as an account ID, Password, Class, Experience, Inventory, Equipped items, etc) I've followed several tutorials on creating, RPG (BurgzergArcade's Hack and slash) but he doesn't really explain whats going on. You follow 200+ videos and then you have a huge system you can't even begin to understand. Gamer2GameDeveloper's FPS tutorial which is a little more informative but doesn't really explain what I need to know, and several other tutorials online.

It seems with all of these, they all come down to creating classes. Like, a player class, item class, and all these other things. I can't seem to find any documentation about what these really are and how they work.

I understand the syntax of C# and the basic methods. Me and a few of my friends want to create a zombie survival game (A little redundant I know) The game will consist of a few player classes to choose from, based off what they were before the outbreak. A soldier, a police officer, Fireman, doctor, athlete, blacksmith, etc. Each class will have different abilities. Then you have weapon classes where the player will gain the ability to use Machine guns, pistols, rifles, melee, archery. NPC's that the player can buy items from, receive missions. A server wide store, with an icon at the bottom saying "Black Market" or something and there are special items players can buy (I will set up all that stuff later though.) There are several other features to the game but these are just a beginning example.

What I need is to find some online docs, or tutorials, that I can research to learn how to put games together, how script classes work and what they do. Just the kind of docs that would supply me with the information I need to achieve this. If anyone can help me please, please do so.

Also, I know it would take forever to put this together, and I don't care so please don't comment just to give some discouraging remarks. I am not asking should I do this, I am just asking if anyone can help provide me with some information.

Thank you for your time, - Josh