By "replica", do you mean a part for restoring an old car or an aftermarket replacement? Replicas are reproductions based on the original design. Many of them are so good most people will never tell the difference. For purists and hobbyists restoring an old car, they can tell the reproduction part by subtle differences.
Aftermarket parts are not intended to fool anyone but they are meant to look and fit just like the original. Often these parts come from the same factory that originally sold them to the manufacturers.
Lower cost and lower quality aftermarket parts can be a good value for older cars but with things like door skins and plastic bumper covers, usually they require more time to make them fit properly. That translates into a higher labor charge, particularly with body shops. A lot of insurance adjusters gain favor with their superiors by convincing owners to accept less expensive aftermarket replacement parts. Many body shops negotiate for a few extra hours to make those parts fit and the adjusters are happy to oblige as long as they can show they saved the company on the cost of parts.