This mystical city lies in K'un-Lun Mountains ( 崑崙山). It's one of the longest mountain chains in Asia, extending more than 3,000 km. The mountain range exists entirely within the Tibet region of the People's Republic of China.
All Immortal Iron Fists come from this city and the Jade Tiger Amulets originate from the K'un-Lun dimension. K'un-Lun is the stronghold of a colony of humanoid aliens, place of origin unknown, whose spaceship crash-landed upon a small, extradimensional world, approximately a million years ago. The ship's warp-drive engines somehow created a permanent, oscillating rift between the pocket world and Earth's dimension, causing the settlement to shift into Earthly space periodically on a site in the remote Himalayan Mountain range in Tibet. The K'un-Lunians built a city around their inverted spaceship, cannibalizing materials from its interior and using its basic design as their architectural style. No citizens alive today are aware that the Central Hall of Ancestors was once the spaceship that bore them to their world. Now that the mystical emerald crystal that regulated the dimensional matrices had been broken (by K'un-Lun's adopted champion Iron Fist), the duration of this period of K'un-Lun's interface with Earth is not known. Apocryphal stories about K'un-Lun told by travelers gave rise to the Chinese legend of the same name. Over a millennium ago, the dragon Chiantang destroyed K'un-Lun in a fit of rage. Recently Chiantang again attacked K'un-Lun, wreaking great devastation and killing most of its people. The rest of the population fled to safety.
The mystical city K'un-Lun appears on the earthly realm only once every ten years.
Netflix's The Immortal Iron Fist
A mystical place that appears every 15 years, Danny Rand was taken in by the monks who lived there. He was trained for years to become the Immortal IronFist. When Danny Rand went back to visit K'un Lun, at the end of the first season, there were Hand bodies lying at the entrance of K'un Lun, however past the entrance, K'un Lun has disappeared.
Danny Rand would return back to New York after his travels to K'un Lun, and later would meet up with Jessica, Matt Murdock, and Luke Cage, to explain to them where K'un Lun is and what K'un Lun is, and that he trained in the way of the IronFist.