In my world, what is the difference between the deep dark realm and the ancient city in my world? You can refer to the following:
The Realm of Darkness will only generate in caves with Y < 0. The surface is covered with a large number of Nether Blocks and Nether Veins, and occasionally generates Nether Sensing Bodies, Netherworld Screaming Bodies, and Netherworld Screaming Bodies. This means that wardens can spawn here.
Ancient cities are generated in the Deep Dark Realm biome, with a fixed generation Y-axis of -52. The main body is made of deep slate etc. and the structure is surrounded by city walls. The center of the ancient city is a large building with the appearance of a statue of the Guardian. There is a redstone circuit in the hidden room. This biome will not spawn any monsters except the Guardian.
