Examine source code of Volume Booster

{
  "manifest_version": 3,
  "name": "Volume Booster",
  "version": "1.0.3",
  "description": "Boost any tab's volume up to 600% with one slider. Remembers the level per site. No tracking, no network calls.",
  "permissions": [
    "storage",
    "activeTab",
    "scripting"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "volume-booster@tiltedlunar123.github.io",
      "strict_min_version": "127.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "action": {
    "default_popup": "popup.html",
    "default_title": "Volume Booster",
    "default_icon": {
      "16": "icons/icon-16.png",
      "32": "icons/icon-32.png",
      "48": "icons/icon-48.png",
      "128": "icons/icon-128.png"
    }
  },
  "icons": {
    "16": "icons/icon-16.png",
    "32": "icons/icon-32.png",
    "48": "icons/icon-48.png",
    "128": "icons/icon-128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "all_frames": true,
      "match_about_blank": true,
      "run_at": "document_start"
    }
  ],
  "commands": {
    "boost-up": {
      "suggested_key": {
        "default": "Alt+Shift+Up"
      },
      "description": "Increase volume by 25%"
    },
    "boost-down": {
      "suggested_key": {
        "default": "Alt+Shift+Down"
      },
      "description": "Decrease volume by 25%"
    },
    "boost-reset": {
      "suggested_key": {
        "default": "Alt+Shift+0"
      },
      "description": "Reset this tab to 100%"
    },
    "boost-mute": {
      "suggested_key": {
        "default": "Alt+Shift+M"
      },
      "description": "Toggle mute"
    }
  }
}