Explain/Disney World Countries
Disney World is essentially... Walt Disney World. In terms of worldbuilding, it is hundreds, maybe even thousands of times bigger than the original resort complex in Florida. All six countries retain their corresponding parks' themes.
Magic Kingdom
The Magic Kingdom was inspired by the park of the same name in Walt Disney World. Its six constituent countries were inspired by the park's six themed lands, albeit with a few name changes—Main Street, U.S.A. is renamed New America and Liberty Square is renamed Libertyland. Adventureland, Frontierland, Fantasyland, and Tomorrowland retain their original names.