The music industry has one great difference which sets it apart from most other industries. The difference is a lack of certification or licenses. Truck drivers, doctors, lawyers, welders, and even massage therapists can show you a piece of paper which says they are qualified to provide a given service. There is no such thing as a certified singer or musician.
So, how do music people find the qualified people they need? If a major star like Garth Brooks needs a new backup singer, guitarist, keyboard player, or whatever, he could run ads in newspapers. Hundreds of people would show up for the auditions. He would spend many days listening. After enough days, he might find the person he wanted. He could do that, but he values his time too much. He would rather have a few very well qualified people show up for the auditions. He would rather spend a few hours listening. He does this by holding private auditions only a few highly selected people know about.
He asks his friends to invite their friends to the private auditions. He knows his band members, lightmen, soundmen, and so forth will only invite qualified people to the auditions. In other words, he gets referrals from people he trusts. He uses people he knows are qualified to screen the people he spends his time auditioning.
If someone wants to determine whether a person is qualified, they can learn through personal experience or they can get referrals from people they trust. You can pick one at random but you won’t know if they are good at what they do until you pay them to do it. Or, you can get a referral from someone you trust.
