It depends on what you are asking. They rely on SAAMI specs for pressure. They all keep their ammo within those guidelines.
As far as bullet weight. They would start with known sectional densities (length vs diameter vs weight). There are known sectional densities that work for a given need (higher SD = deeper penetration).
As far as powder charge. They would play with in known burn rates and experiment til they found a powder/charge weight that achieved the desired velocity and was within SAAMI pressure specs.
Bullet design is a whole other animal. Most jacketed ammo uses a lead core that is pressed into a copper jacket. The jackets are made in fixed thicknesses with some degree of varience to be expected. For FMJ ammo that is not a big deal as they are interested only in getting the bullet to the target with reasonable accuracy. For hollow point pistol ammo it another thing entirely. They must try to achieve uniform thickness of the jacket and manipulate the design and dimensions of the hollow point to achieve reliable expansion. The Speer Gold Dot is the glaring exception. They can start with a predetermined velocity and hollow point design and adjust the thickness of the jacket to get the desired results. The Gold Dot is a plated bullet so they can plate it as thick, or thin as they need to. Also because the jacket is directly bonded to the core, one molecule at a time, they can virtually guarantee 100% weight retention.