Entity is the base class for pretty much everything in SketchUp. But since C doesn’t support classes and inheritance we need functions like
SUComponentDefinitionToEntity to cast between types. You would use
SUComponentDefinitionToEntity when you need to call any of the functions in
Entities are containers for Entity objects. The model has a collection - this is the root of the model hierarchy. And each component definition has an Entities collection.
That depend on the type of Entity. Edge, Faces, Groups etc - all of them are of type Entity.
Here’s an old chart - but it shows the gist of the object hierarchy: