Thanks Nicolò, your feedback was very useful!

So it's not program-not-found but command-not-found and it's define
here:

  https://github.com/NixOS/nixpkgs/tree/master/nixos/modules/programs/command-not-found

Then I found this

  https://github.com/NixOS/nixos-channel-scripts

All this is pretty clear and simple.

The main thing I wonder at this point is when the
"generate-programs-index.cc" file is run.

What would be the good entry point in substitute servers to populate
such a database?

-- 
Pierre Neidhardt
https://ambrevar.xyz/