C# :: How Can A Method Work With Just A Declaration

I want to access the body of the Add() of a list in c# to see how it works, but it only just gives me the declaration.

[DebuggerTypeProxy(typeof (Mscorlib_CollectionDebugView<>))]
[DebuggerDisplay("Count = {Count}")]
[Serializable]
public class List<T> : IList<T>, ICollection<T>, IList, ICollection, IReadOnlyList<T>, IReadOnlyCollection<T>, IEnumerable<T>, IEnumerable {
public void Add(T item); // thats all. I tried go to declaration but still gives me this line of code. This is from metadata in Visual studio.
}

How this thing work. It just a declaration not a definition yet its still doing something. How is that possible.