It's called mold making or casting. Pulling a thin flexible plastic over a mold is often done with vacuum forming. All can be done at home using conventional ma

It's called mold making or casting. Pulling a thin flexible plastic over a mold is often done with vacuum forming. All can be done at home using conventional materials and equipment. This is a traditional craft and sculpture technique. I've never done it myself, but there will be plenty of books on it, probably meetups in your area, and certainly tutorials on cosplay and prop replica sites, and on aggregators like Instructables.