My guess is large amounts of Resource extraction of pre-war resources from shampoo, paper,scrap metals, books, laundry detergent, drugs, cobblestone from bricks, wheels, wood and cloth ect. These are excavated in organized government excavation projects and paid for with food grown from the countryside. Commuting is common from the countryside though suburbs with some level of food production capacity(like North Korea any land that can be farmed by survivors likely is) could form permanent settlements and centers of trade.