Your post is not clear.. If you take your car to a dealership, they will normally select dealer OEm parts, and you will pay through the nose for them.. AND they may not be as good as, nor any better than, what you could have bought at your friendly local auto parts house.
If you go to an independent, he will normally choose a part from a supplier (NAPA, for example) which he has found to be reliable, and which is not likely to cause him an expensive return under warranty.
You have no guarantee, unless you discuss it with the shop before the work starts.
OEM parts are not necessarily the best that exist. And aftermarket can be either good or bad. You have to KNOW what you are getting.