The IObjectManager51 is the interface that provides the methods to create all the different types of objects, locations and routes in the project. Using one of the Create methods (i.e. IObjectManager51::CreatePyramid() ) you can add the objects to the project and place them in any location in the 3D World. When the client application creates such an object, it gets an interface to the object in return, which can then be used to perform further operations on it.
See: “Adding objects to the project” for an example code in C++ and “Creating Polyline using safe array of vertices” for an example code in VBScript