For Ports, the grapes are picked extremely ripe, then crushed in the same manner as are red wines. Yeast is added to the must and fermentation begins. The must is constantly punched down to get as much color from the skins as possible. When fermentation is about halfway complete, neutral spirits are added. What you are left with is a sweet, very alcoholic dessert wine.
Sherries have alcohol added after fermentation is complete.
Any sweetness is added after fortification.