zombiiattack-datapack/data/zombiiattack/dimension/lobby.json

{
  "type": "zombiiattack:lobby",
  "generator": {
    "type": "minecraft:flat",
    "settings": {
      "biome": "minecraft:plains",
      "features": false,
      "lakes": false,
      "structure_overrides": [],
      "layers": [
        {
          "block": "minecraft:air",
          "height": 1
        }
      ]
    }
  }
}