Examine source code of Sender Audit for Gmail

{
  "manifest_version": 3,
  "name": "Sender Audit",
  "browser_specific_settings": {
    "gecko": {
      "id": "senderaudit-for-gmail@senderaudit.com",
      "strict_min_version": "109.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ],
        "optional": []
      }
    }
  },
  "version": "1.2.0",
  "description": "Analyse emails received in your webmail with Sender Audit (SPF, DKIM, DMARC, IP reputation, spam score…).",
  "permissions": [
    "storage"
  ],
  "host_permissions": [
    "https://mail.google.com/",
    "https://senderaudit.com/"
  ],
  "icons": {
    "16": "icon16.png",
    "32": "icon32.png",
    "48": "icon48.png",
    "128": "icon128.png"
  },
  "action": {
    "default_popup": "popup.html",
    "default_title": "Sender Audit",
    "default_icon": {
      "16": "icon16.png",
      "32": "icon32.png",
      "48": "icon48.png",
      "128": "icon128.png"
    }
  },
  "background": {
    "service_worker": "background.js",
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "https://mail.google.com/*"
      ],
      "js": [
        "providers/base.js",
        "providers/gmail/content.js",
        "content.js"
      ],
      "css": [
        "styles.css"
      ],
      "run_at": "document_idle"
    },
    {
      "matches": [
        "https://senderaudit.com/extension-welcome.html*"
      ],
      "js": [
        "welcome-relay.js"
      ],
      "run_at": "document_start"
    }
  ],
  "web_accessible_resources": [
    {
      "resources": [
        "providers/gmail/inject.js"
      ],
      "matches": [
        "https://mail.google.com/*"
      ]
    }
  ],
  "externally_connectable": {
    "matches": [
      "https://senderaudit.com/*"
    ]
  }
}