Unreal Engine is mainly written in c++. But there are some things that are written in c#.

1> So what functionalities are written in c#?

2> And also are there any benefits of using c# instead of c++ in those cases?

And since unreal engine developers often have panel discussions and forums or documentations detailing their decisions. It would be great if someone can provide a link to them discussing or detailing why they used c# over c++ for implementing those features.

unrealEngine screenshot with .cs search results

https://github.com/EpicGames/UnrealEngine