minecraft-assets/assets/minecraft/blockstates/cauldron.json

{
  "variants": {
    "level=0": {
      "model": "minecraft:block/cauldron"
    },
    "level=1": {
      "model": "minecraft:block/cauldron_level1"
    },
    "level=2": {
      "model": "minecraft:block/cauldron_level2"
    },
    "level=3": {
      "model": "minecraft:block/cauldron_level3"
    }
  }
}