Before the CLR was open sourced you could investigate how things were implemented by using decompilers on the MSIL code or later shared source, but but investgations often stopped at a InternalCall just as things started to get interesting. Now you can see the whole story of how abstractions are implemented part in C# and part in C++ when the runtime needs to "bend the rules" of what C# would normally allow. The corert[0] project is interesting because a larger portion of the run is implemented in C# (or as compilation passes instead of runtime code generation). For example, you can find exception dispatch and type casting implemented in C# there [1].

One example from the article that may be a little off is getting the length of string. It looks like the call to string.get_Length is probably replaced with a JIT intrinsic[2]. The importer then turns that intrinsic call into a GT_ARR_LENGTH[3] node that is eventually lowered into something like[4]:

    *(stringAddr + offsetOfStringLengthField)

It's a bit more complicated than that, as there are several references to GT_ARR_LENGTH in various optimization passes.

[0]: https://github.com/dotnet/corert/ [1]: https://github.com/dotnet/corert/tree/master/src/Runtime.Bas... [2]: https://github.com/dotnet/coreclr/blob/a123c3972c447c8bcfa18... [3]: (warning, big file) https://github.com/dotnet/coreclr/blob/a123c3972c447c8bcfa18... [4]: (warning, big file) https://github.com/dotnet/coreclr/blob/a123c3972c447c8bcfa18...