idk
I have some Haxe coding experience due to the engine, so I can help you with some things.
Song Files
.json files are the type of the file your chart is saved on when you save the chart.
Story Week and Song List
You can change what songs are in which week, and even delete a week altogether. I'll let you try to understand whats going on in both versions of the file.
Freeplay Songlist
This is easy. Just enter your custom song's name with a capital letter at the start of every word. After doing that, you can play it in the Freeplay Menu. It is utilized in every engine, even in the original game.
That's just basic knowledge. Things like Modcharts I can't figure out yet, but I hope this helped by giving you more info about the engine.