My first job out of high school was as a parts counter kid. I had some knowledge before coming in, but I had a whole lot to learn once I started. I think the key was the willingness to admit to yourself, and the customer when you didn't know something, then find the resources to get the right answer.
Sometimes, it was one of the old guys you worked with who could give you the answer. Sometimes, it took diving into the paper catalogs. I worked parts in the late 90's, early 00's, so the paper catalogs were still very necessary. None of the computer systems had the pictures in them yet, and everything was still DOS based interface- keystrokes only, no mouse.
I lived in a military town, so all the Marines had their favorite project cars, and after a while, I was the guy they came to, because I would take the time to get the right part for that old Malibu or find the weird brake drum for that '72 K20. That brake drum took hours to find, and I had to get the original first to take measurements. I think it was a limited run brake system. And those drums, even back then, cost over $80 each.
The other day, I needed a bearing for a pulley that there was no listing for, and I asked a guy for the bearing / seal book so I could spec it out. I had replaced another on the same car- different pulley, so I knew it was a common bearing, but I didn't keep the part #. They didn't even have the book in the store. I really don't think the corporations really care enough to make sure the customers get what they need anymore. They just want to pump out the high volume parts & accessories.