Learn French: Le chef fait sélectionner un dossier parmi le tas. - The boss is having a file selected from the pile.

Parmi means "among" or "from" and is used when referring to a larger, mixed group or a collective whole, like a pile (le tas). If the boss were choosing between specific, explicitly mentioned items, you would use entre (for example, entre ces deux dossiers - "between these two files").

Why use the structure fait sélectionner instead of just sélectionne?

This uses the causative construction (faire followed by an infinitive). It means the boss isn't physically picking the file himself; he is having someone else do it. If he were doing the action himself, it would simply be Le chef sélectionne un dossier.

The English translation is "is having a file selected." Why isn't it translated passively in French, like fait être sélectionné?

French never puts the second verb in the passive voice in a faire construction. When you don't mention who is performing the action, faire + active infinitive naturally translates to "having [something] [past participle]" in English.

Does le chef mean the boss of a company or a cook in a restaurant?