Comment by 0xDEF

They use their own heavily optimized Mono runtime that is .Net Standard 2.1 compliant and supports all C# 8 and many C# 9 features.

As a C#/.Net developer I'm satisfied with the C# and .Net support in Unity. The pain points are elsewhere.

https://docs.unity3d.com/Manual/dotnetProfileSupport.html

https://docs.unity3d.com/Manual/CSharpCompiler.html