The purpose of the selection procedures are to ensure that very few candidates fail subsequently. In my experience very few do.
The selection process itself is a pyramid. A lot apply and that gets whittled down to a manageable number to be interviewed and or assessed, and that is further whittled down to the successful number that are offered employment. The end number is going to depend on the particular airlines requirement at any given time.
For airlines with cadet schemes, there are similarly going to a specific number of opportunities and the candidates training record will provide much of the background provenance. The selected candidates will also go through an interview process with successful candidates joining the airline on a probationary contract in accordance with the terms of that scheme.