To answer your question...a nurse externship can vary from student nurses working in a CNA type role, to student nurses basically being paired with a registered nurse and getting to help with nursing skills that they are legally allowed to perform. I've never been in one, I just worked as a CNA/tech as a student. I have seen externships offered in various specialties. It seems like a good way for a student to get their foot in the door. These are for students who have not graduated nursing school
yet, and often are one year from graduation.
A nurse residency is usually for new grad RNs to orient them to the RN role and help them transition from student nurses to registered nurses. Some hospitals have year long residency programs that pair classroom teaching with orientation on the floor.
I don't know the transition process as I am not familiar with that hospital, but I would assume that if you started as a extern, then got hired as an RN, you would still go through the normal orientation/residency program for new grads since you would still be a new grad...who had the opportunity to already work at the hospital.
Hope this helps!