Short answers, pulled from the story.
Michael J. Cullen, a former Kroger employee, opened the first true supermarket in the United States on the 4th of August 1930. The store, King Kullen, was located inside a 6,000-square-foot former garage in Jamaica, Queens, New York City. It operated on the principle of "pile it high and sell it cheap" and was the first to combine self-service, separate departments, discount pricing, and volume selling.
International expansion accelerated after the U.S. Department of Agriculture presented an "American Way exhibit" at the International Food Congress in Rome, Italy in 1956, featuring the first fully stocked supermarket outside the United States. The following year, Supermarket USA at the Zagreb International Trade Fair in Yugoslavia showcased 4,000 consumer items in a 10,000-square-foot exhibit. Both events were used as Cold War propaganda to demonstrate Western abundance.
Before supermarkets, customers stood in front of a counter while clerks fetched items from shelves behind them. Prices were not posted, requiring shoppers to haggle with staff. Stores carried as few as 450 items, and a complete weekly shop required separate visits to a greengrocer, butcher, bakery, fishmonger, and dry goods store. American urban families in the 1920s spent fully one-third of their household budget on food.
Clarence Saunders introduced self-service grocery shopping at his Piggly Wiggly stores, the first of which opened in 1916. Saunders was awarded several patents for his innovations and began offering franchises after the stores proved financially successful. Michael J. Cullen built on this foundation by adding separate food departments, discount pricing, and a parking lot when he opened King Kullen in 1930.
The spread of supermarkets drove the share of disposable income that American consumers spent on food from twenty-one percent in 1930 down to sixteen percent in 1940. Before the twentieth century, food was neither cheap nor abundant; in the 1920s the average urban family spent one-third of its total budget on food. The modern era of cheap food had begun by 1940.
Supermarkets use circulation, coordination, and consumer convenience as core layout principles. Fresh produce is placed at the front to signal health; bread and milk are placed at the rear to force shoppers through the full store. High-impulse and high-margin items occupy the most prominent positions, cheap generic brands sit on the lowest shelves, and end-of-aisle displays are sold to manufacturers as premium placement. Small candy, magazine, and drink displays at checkout encourage last-minute purchases.