How is the SAMD section in https://downloads.arduino.cc/packages/package_index.json being created/where are the sources?

I'm looking for the original sources of the "Arduino SAMD Boards (32-bits ARM Cortex-M0+)" v1.6.8 boards section in the package index https://downloads.arduino.cc/packages/package_index.json but am unable to find it.
It seems there are some old templates in this repo in https://github.com/arduino/ArduinoCore-samd/tree/703b87e02d1ce61cd3dddcaa7c4f9ff50c537f80/extras but from comparing it seems like they are (no longer?) used.
Where does the board section come from?

Start by comparing the ArduinoCore-samd extras templates at the referenced commit with the Arduino package_index.json SAMD section for version 1.6.8. Trace any referenced packaging or release entry points, then document where that section is generated and which source files are authoritative.

Written by the indexing model from the issue text.