Examine source code of YTSync

{
  "manifest_version": 2,
  "name": "YTSync",
  "description": "Watch videos directly on YouTube with your friends.",
  "author": "Tandashi",
  "version": "2.0.1",
  "icons": {
    "16": "icon16.png",
    "32": "icon32.png",
    "48": "icon48.png",
    "128": "icon128.png"
  },
  "homepage_url": "https://github.com/Tandashi/YTSync-Plugin",
  "web_accessible_resources": [
    "js/jquery-3.4.1.min.js",
    "js/socket.io.slim.js",
    "lib.user.js"
  ],
  "content_scripts": [
    {
      "matches": [
        "https://www.youtube.com/",
        "https://www.youtube.com/feed*",
        "https://www.youtube.com/watch*"
      ],
      "js": [
        "extension.js"
      ],
      "run_at": "document_end"
    }
  ]
}