Well, it must be very few experienced and rated pilots since the application was not open for all!
No, fmgc is correct. For many years now it has been a combination of cadets from an extremely good tried and trusted source, together with a roughly equal mix of experienced pilots.
As you would expect, cadet entry pilots place heavy demands on the number of training tracks that are available. Experienced pilots less so, and experienced type rated pilots, very few.
I am not sure where you get the idea that the vacancies available are "not open to all"? The fleet office is awash with applications, and more arrive with every mail delivery. It is always a case of sending in a current CV, and as so often in life, it is sometimes simply a case of timing.
So I am afraid you are wrong on both counts.