If grocery stores close, the merchandise is purchased by Dollar Stores. Also, they get close-out products, odd flavors and the occasional box of stuff with Spanish lettering rather than English.
The food is just the same as the regular store. Just check the expiration dates. Cold food such as meats, cheeses and frozen dinners are usually the result of store, convenience store or discount store close outs. If the date is good, the food is edible.
The also get overstocks. If a store or a manufacturer has a surplus, they'll sell it to Dollar Stores to get rid of it. Better to get some money for it than no money.