turbo.json files can be nest in packages and apps as long as they extend the base configuration.

{
  "extends": ["//"],
  "tasks": {
    "build": {
      "dependsOn": ["^build"],
      "env": ["API_KEY"],
      "inputs": ["$TURBO_DEFAULT$", ".env*"],
      "outputs": [".next/**", "!.next/cache/**"]
    }
  }
}

I definitely prefer this approach to sticking everything in the single turbo.json as I can easily bin off old packages / apps without their configurations lingering.