More details can be found here: https://news.ycombinator.com/item?id=40918052

Basically it's an old google extension, enabled by default on google chrome, edge and brave and only available on *.google.com urls.

Extension's source code can be found here: https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/resources/hangout_services/thunk.js