Short answers, pulled from the story.
The word hamburger traces back to Hamburg, Germany. The term originally derives from the city name before linguistic rebracketing made burger a standalone word for various sandwiches containing different meats.
Multiple individuals and locations claim credit for inventing the first commercial hamburger. Louis Lassen opened Louis Lunch in New Haven around 1895, while Charlie Nagreen sold a meatball between bread at the Seymour Fair in 1885.
White Castle opened its first restaurant in Wichita, Kansas, in 1921. This location became the first major chain to sell hamburgers and introduced small square patties called sliders.
Regional adaptations include the Takumi Burger in Japan which costs 1,000 yen and features avocado and fresh wasabi. Denmark introduced the bøfsandwich in 1949 with beef and horse meat, while Australia and New Zealand feature plain hamburgers known as burger with the lot.
Vegetarian and vegan burgers emerged as meat analogues formed from tofu, TVP, seitan, beans, grains, or vegetables starting in the 1890s. Companies like Loma Linda, Boca Burger, and Lightlife began producing soy-based burgers during the 1960s and 1970s.
The first hamburger grown with cow cells outside a living animal was served on the 5th of August 2013. This result came from a $300,000 research project into cultured meat led by Mark Post at Maastricht University.