Examine source code of Watchmarker for Youtube

{
  "manifest_version": 3,
  "name": "Watchmarker for Youtube",
  "description": "Automatically mark videos on Youtube that you have already watched.",
  "homepage_url": "https://sniklaus.com/",
  "version": "5.1.0",
  "permissions": [
    "alarms",
    "cookies",
    "declarativeNetRequest",
    "declarativeNetRequestWithHostAccess",
    "history",
    "scripting",
    "storage",
    "tabs",
    "webRequest",
    "webRequestBlocking"
  ],
  "host_permissions": [
    "*://www.youtube.com/*",
    "*://m.youtube.com/*"
  ],
  "action": {
    "default_title": "Watchmarker for Youtube",
    "default_icon": {
      "72": "content/icon.png"
    }
  },
  "options_ui": {
    "page": "content/index.html",
    "open_in_tab": true
  },
  "icons": {
    "72": "content/icon.png"
  },
  "background": {
    "scripts": [
      "idb.js",
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "*://www.youtube.com/*",
        "*://m.youtube.com/*"
      ],
      "css": [],
      "js": [
        "youtube.js"
      ],
      "run_at": "document_start",
      "world": "ISOLATED"
    },
    {
      "matches": [
        "*://www.youtube.com/*",
        "*://m.youtube.com/*"
      ],
      "css": [],
      "js": [
        "hooks.js"
      ],
      "run_at": "document_start",
      "world": "MAIN"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "yourect@coderect.com"
    }
  }
}