Until now first people had to pass a tough selection process and then, and only then, if they were the top of the stack they would be picked to be trained
In my understanding (which is admittedly incomplete) that is/was correct. However, the selection process was a
civil service selection process, rather than one specifically geared to identify the best people for the job at hand--in other words, whoever studied harder the more or less relevant curriculum (or had the best connections, as pointed out above) got the job. However, it is doubtful whether the person who can answer correctly a question about article 15 of the Spanish Constitution, or who knows the Chicago Convention by heart will make the best controller.
As an anecdote, we used to have fun in Spain by asking the controller unusual (but perfectly legitimate) questions and try to guess what nonsensical answer we would get. My favourite was "request activation status of danger area LE-XXX"--typical answers were "affirm, you are cleared to FLXXX", "contact Marseille on ...." (wrong country, my friend!), or the all-time popular "say again?"