Unity dreams of removing all the C++ code and replacing it with their "High Performance c#", ie. using the burst compiler and manual memory management to remove the GC problem.

Unreal Engine uses C++ for both.

No, Unity has already replaced a few C++ modules with HPC#, their C# subset for low level coding.

They also plan to eventually use NativeAOT as part of the .NET Core migration to further go down that path.

Unity has been one of the reasons why C# has gotten so much better for low level coding since version 7, alongside feedback from Midori experience and wanting to win Techempower benchmarks.