Unity uses aginging Mono runtime, because of politics with Xamarin, before its acquisition by Microsoft, migration to .NET Core is still in process.

Additionally they have HPC#, which is a C# subset for high performance code, used by the DOTS subsystem.

Many people mistake their C# experience in Unity, with what the state of the art in .NET world is.

Read the great deep dive blog posts from Stephen Toub on Microsoft DevBlogs on each .NET Core release since version 5.0.