One of the basic processes in wet-forming vegetable-tanned leather is called "boning". A smooth and non-abrasive tool (originally bone, antler, or similar) is used to mold and sculpt the leather into the desired shape.
In more modern times the tools used are more frequently plastics or hardwood pieces shaped to achieve specific results.
This work is always performed by hand, and it requires both strength and precision in applying pressures to achieve the desired result. Most modern manufacturers have long since gone to hydraulic or pneumatic presses to form holsters to the specific shape of a handgun. Only the custom shops or limited production shops continue to provide genuinely hand-boned holsters.
After 43 years of doing that work I can tell you that it takes a toll on the joints, ligaments, and tendons, and arthritis reminds me every day about the stresses and strains I experienced while producing my products.