I live across the creek from a large hatchery, the Whitman Lake Hatchery at Herring Cove. I previously lived near the Deer Mountain Hatchery and use to help when the fingerlings were released back into Ketchikan Creek. 
When the wild salmon go up the creeks, a certain number of them are caught and the eggs are extracted from the females and the males are "milked" of sperm. Then both are very carefully "mixed" together with hopes of fertilization. 
Once they hatch and reach the correct size, they are released back into the water(creek)  and swim they into the Pacific Ocean or sea water to live (and die) until it's time to spawn. During the time they are in the Pacific, they eat whatever salmon eat.
When it's time to return to spawn, they return to the same creek that they came from and that their parents went up to. They are NOT fed or given steroids or other growth hormones. They face the same conditions all wild salmon do, nothing is done to change their biological characteristics.