help needed to grasp basic concepts in data type conversion

when writing c# script in GH, I know how to handle following data types.

Curve, Mesh, Brep, int, bool, double.....

but for example, when trying to reference curve objects in rhino document, I have learned that I have to write something like following,

      List<Curve> crvs = new List<Curve>();
      List<double> lengths = new List<Double>();

      foreach(Rhino.DocObjects.RhinoObject obj in doc.Objects.GetObjectList(Rhino.DocObjects.ObjectType.Curve)){

        GH_Curve crv = new GH_Curve();
        crv.CastFrom(obj.Geometry);
        crvs.Add(crv.Value);
        lengths.Add(crv.Value.GetLength());

        }

Q1.
how is Rhino.DocObjects.RhinoObject different from grasshopper usual data types(Curve, Mesh, Brep, int, bool, double...)?

Q2.

I do not undetstand exact concept of 'Casting'. how is 'CastFrom' different from .ConvertToXXX, or .ToXXX?

Q3.

how is GH_Curve different from Curve data type?

Q4

When I search Grasshopper3D for similar topics, I often see GemetryBase, GeometricGoo..... What are these?

Sorry in advance that my coding proficiency is very frustrating...