Marvel Comics
Marvel counts among its characters such well-known superheroes as Spider-Man, Iron Man, Captain America, Wolverine, Thor, Hulk, Ant-Man, such teams as the Avengers, the Guardians of the Galaxy, the Fantastic Four, the Inhumans and the X-Men, and antagonists such as Doctor Doom, The Enchantress, Green Goblin, Ultron, Doctor Octopus, Thanos, Magneto and Loki. Most of Marvel's fictional characters operate in a single reality known as the Marvel Universe, with locations that mirror real-life cities. Characters such as Spider-Man, the Fantastic Four, the Avengers, Daredevil and Doctor Strange are based in New York City, whereas the X-Men have historically been based in Salem Center, New York and Hulk's stories often have been set in the American Southwest.

Q: Captain America, Thor and Ultron all inhabit what fictional milieu?
Marvel Universe