About how freecodecamp website was built

I am wondering what technologies freecodecamp used to build a “what you see is what you get” coding interface and how it check the codes correctness for a long time.
I tried to look into the source code or inspect the elements of a webpage, but I turned out understand hardly about how it work as I don’t have much knowledge in web programming.
Can someone share me about how do the editing, output and viewing frame work?

You can see the whole codebase on our GitHub repo.