It turns out Google Chrome (via Chromium) includes a default extension which makes extra services available to code running on the `*.google.com` domains - tweeted about today [by Luca Casonato](https://twitter.com/lcasdev/status/1810696257137959018), …
This is included in the chromium source code which is public
https://github.com/chromium/chromium/blob/128.0.6586.1/chrome/browser/resources/hangout_services/thunk.js