Implementation supplied functions are part of the hosting implementation, although it may depend on the core functionality of the interpreter to provide the necessary services.
The source text for an implementation supplied function is provided by the host environment. The mechanisms by which they are created is host dependant.
The functions created by the implementation may have any combination of ImplicitThis and ImplicitParents attributes. These control the way that the scope chain is set up when the function is initialized. Note that these are function object attributes and notProperty attributes. Depending on the host implementation you may or may not have access to these attributes to be able to define their settings.