Unless the photos you're thinking of were retouched, whatever you see reflected in the ornaments is the photographer's setup. The trick is to make that look like something other than a photography studio. That means two things: 1) provide the environment that you want reflected, and 2) hide or camouflage the things (like the camera) that you don't want to see. To accomplish that, you can set up some props such as a bowl of fruit, a vase of flowers, pine branches, etc. to create the environment you want. A studio soft light can be modified to look like a window, or you can just use a real window for light. The camera can be hidden among the props or concealed (except for the lens, of course) behind some black cloth so that it blends into the scene, and a remote trigger can be used to keep the photographer out of the scene.

A long lens can help, too: with spherical objects like Christmas tree balls, reflected objects become tiny very quickly as distance increases. For example, using a 100mm macro lens instead of a 50mm macro lets you get the camera twice as far away from the subject, and that makes the camera's reflection much smaller.