"I'm also not sure if a car mechanic needs to know how an engine works. I'm assuming almost none of them could design a car engine from scratch. They know just enough to know which parts needs to be replaced."

That's why, when your car has a problem, a lot of mechanics just blindly replace parts with the hope that something will fix it. You are much better off with a mechanic that understands how the car works. And you will save a lot of money.