A number of things happen to the props. 1. If they are "hero" props (those used by an actor in a scene), then they are often stored in case of a re-shoot. 2. If there are multiples of an item they can be stored, sold, or given to cast & crew as parting gifts, etc. Sometimes, especially if they are rare or difficult to build, etc. then they are given to another production or kept by the company and used again.