How do dentist make molds without teeth for dentures/implants?

Straight forward: They will have a teeth try in to check the height of your bite, shape of the teeth, esthetics and function. Once this i all finalize then they will send the denture in for processing to make the final product.

Impressions.: Molds, or impressions, are used to make dentures and to make restorations on implants. There are various materials available that are reasonably accurate. Whatever the material, it is placed in a tray shaped to fit around the upper or lower teeth or ridge and then seated in the mouth until it hardens. Dental plaster is mixed and vibrated into the impression, hardened and removed- a model of you!

Same as with teeth: Usually we make a preliminary impression (mold) in quick setting alginate material. We then use the cast made from this to make a special customized tray just for your mouth. Then we make a final impression with a different material in the custom tray. This provides more information and is very accurate. The cast from this impression is used in the making of the dentures, with or without implants.

Dental magic!: All kidding aside, after making impressions of your gums and measurements of your jaw relationships, dentists can determine the size and position of teeth from the anatomy of your mouth, xrays, and facial features.