The Japanese returnees opened yatai, or mobile food stalls, while serving ramen and gyōza. These stalls became important informal food businesses in postwar Japan, where vendors could serve inexpensive hot meals with limited equipment.
A yatai typically consists of a wheeled cart or compact stand that opens in the evening and is removed late at night or early in the morning. Ramen was a signature offering, and gyōza made a practical side dish: they could be prepared quickly, pan-fried to order, and served alongside noodles.
The broader history of yatai reaches back much further than the postwar gyōza story. Food stalls existed in Japan centuries ago, and mobile stalls became especially widespread during the Meiji period and after World War II. In the modern era, regulations and public-health concerns reduced their numbers in many cities.
The alternatives describe different things: an onsen is a hot spring, a ryokan is a traditional inn, and an izakaya is a Japanese drinking-and-dining establishment. None means a mobile food stall.