Where is the aircraft registration?

When going over the raw data that is used in FlightAware (json files) I do not find the aircraft registration names.
Still in the table this information is shown.
Drilling through the different code I am not (yet) able to find where this data is being collected.
Could someone point me in the right direction on how this data is being found.
I would like to (re)use this to create some alerts for a local flight school to have their own planes being pushed as notification to members. (push notification in their own app)

thanks in advance!