Where Is the Join Code Civ 6?

When you create a game, either on steam (unified mode) or on epic you get a join code. use this, there is a green join button in the internet game menu in the top right.

What is civ6 coded in?

C++, but the devs want you to stick to the LUA + XML that they've exposed, same as last year. You'll find that they've used "Kinds" to add an extra level of depth to the modding community, think this might have potential to more easily add your own modifiers, but we'll see.

How do you join a private game in Civ 6?

Go to multiplayer, internet, then in the lobby you can host a game. In the settings you can make it private. Once you have the settings like you want just go to steam and invite your friends from there.