minecraft-assets/data/minecraft/worldgen/configured_feature/trees_windswept_hills.json

{
  "type": "minecraft:random_selector",
  "config": {
    "default": "minecraft:oak_checked",
    "features": [
      {
        "chance": 0.666,
        "feature": "minecraft:spruce_checked"
      },
      {
        "chance": 0.1,
        "feature": "minecraft:fancy_oak_checked"
      }
    ]
  }
}