So what you should be seeing is “Solution ‘Your Project Name’”. Under that, you should see a folder titled “engine”. That folder contains both source related to gameplay developement and, I believe, the editor, yes. Below that, you should see “games”. This is where you source should be, assuming you have any. It would be under whatever you titled your project.
If you don’t see your project’s source code, you might check which of the two FPS templates you chose. One is blueprint-based and has little or no code in it. I don’t remember which, offhand, but it’s worth double-checking.
If you’re positive you’re using the C++ template, check the C++ content folder. If you see some classes there, then you may just need to generate your C++ project. To do that, go to the “file” drop-down menu and select the option under “project”.
As for your question about github, you don’t ever need to touch it unless you want to work on the engine’s source code and promote it beyond your own use. I might be wrong, but I don’t think you’re planning on that yet, so you can safely ignore github.