Ancient cities generate in the deep dark biome, which is always underneath mountain biomes.
To find ancient cities, mine down to y-level -52 in the deep dark biome underneath a mountain biome.
Ancient cities have a central structure that resembles a portal and contains treasure rooms.
Use wool blocks to block skulk sensors and avoid triggering shriekers in the ancient cities.
Flying with elytra and drinking night vision potions can help you navigate and avoid the warden in the ancient cities.