Opera companies are commercial institutions like any others, with perhaps the exception of government maintained instances, and they will choose the singer based on the result achieved *on the singing*, no matter how many years of training, technical demonstrations or certificates are presented (even in the case of the govnt maintained ones, for the lead roles hardly they will do something different).
Some allow volunteering, which of course will be for minor roles but will allow the person to get in. If you take an audition for a role, you will be asked to sing a couple of pieces, the conductor can also test you to see how you work, but the nature will be practical because well, they want to get someone good for the show so that they make money out of it.
Granted that for someone in Opera learning technique would probably be a good idea, it´s just a tool, and not the goal. Some of the most acclaimed tenors had no training when they started their carreers. Many have radically different backgrounds. The common part is that they could/can sing the living hell out of their parts.
Opera has a focus on virtuose, yes, and that´s why technique is probably a good idea if you are not some sort of genius, but you can find the same in many genres and it won´t prevent you from performing lesser roles IF you can perform the piece of course.