Although the Quantum Realm has played a role in several Marvel Cinematic Universe films, it still remains largely a mystery. However, concept art for Ant-Man and the Wasp reveals a better look at the Quantum Realm's Microverse City.
Artist Jackson Sze posted the art on Instagram. According to Sze, the art was made for Ant-Man and the Wasp, with the city appearing as Hank and Janet leave the Quantum Realm near the climax of the film. Sze writes, "I wonder if Scott Lang made it here during his unscheduled stay in the Quantum Realm."
There's no indication Scott actually made it to Microverse City. However, Janet van Dyne survived there for several years, so it's entirely possible she got to see it at some point.
Although not exactly the same, the MCU's Quantum Realm is very similar to the Microverse of the Marvel Comics. In the comics, the Microverse is home to many characters, including Bug, who eventually becomes a Guardian of the Galaxy. The area has also played a role in several comics events One notable event using the Microverse is Minimum Carnage, which involved Spider-Man's titular foe being used as a tiny, swarming replicated force of evil.
Directed by Joe and Anthony Russo, Avengers: Endgame stars Robert Downey Jr. as Iron Man, Chris Evans as Captain America, Mark Ruffalo as Bruce Banner, Chris Hemsworth as Thor, Scarlett Johansson as Black Widow, Jeremy Renner as Hawkeye, Brie Larson as Captain Marvel, Paul Rudd as Ant-Man, Don Cheadle as War Machine, Karen Gillan as Nebula, Danai Gurira as Okoye and Bradley Cooper as Rocket, with Gwyneth Paltrow Pepper Potts, Jon Favreau as Happy Hogan, Benedict Wong as Wong, Tessa Thompson as Valkyrie and Josh Brolin as Thanos. The film is in theaters now.