Lost with FCC-CodePen-GitHub-Repl.it

I’m a new user and I’m lost between the different use of FCC with CodePen, GitHub and Repl.it .

Now I’m using CodePen to make the projects. I understand that CodePen is used to have an environnement where I can create and save the project. But GitHub and Repl ? What for ?
I was thinking GitHub can be use to save the code of the different challenge but I failed unterstand how it’s works and link FCC and GitHub.
Well, I’m lost.

Try reading this