Learn Spanish (Latin America): Durante la audición, se selecciona a los mejores modelos. - During the audition, the best models are selected.

This is the core of using the impersonal se with the personal a. When we talk about people receiving an action, we add the personal a to avoid confusion. Once you add that a, the phrase a los mejores modelos acts as the direct object of the sentence, not the subject. Because the sentence no longer has a plural subject, the verb must default to the singular selecciona.

What would it mean if we dropped the a and wrote se seleccionan los mejores modelos?

Without the personal a, the sentence could become a standard reflexive or reciprocal construction. Because people are involved, it creates ambiguity. A listener might interpret it as 'the best models select themselves' or 'the best models select each other'. Using se selecciona a makes it entirely clear that the models are the ones being selected by an unnamed group.

Is the word modelo always masculine because it ends in -o?

No, modelo is an invariable noun when referring to a profession, meaning the word itself doesn't change based on gender. You use el modelo for a male model and la modelo for a female model. Here, los mejores modelos refers either to a mixed group of men and women, or to a group of exclusively male models.

Why use durante here instead of to say 'during'?