The point of all of this is that it is different at each and every site, and although there are general trends at each and every site, it isn't predictable. No one knows what attrition will be, or how many upgrade to license, etc to predict the make up of the next class.
The fact is, a utility will try to hire internal first, but lacking enough interest, will post to the public. From there, they will generally hire a mix of navy nukes, college grads and other industrial experience.
It is broad and general. All you can do is watch the job boards.