How does FlightAware correlate flights to Part 91 flight plan data? I'm looking at some flights in FlightAware. With the scheduled air carrier flights, their call-sign can be crossreferenced with a flight schedule database to determine the origin and destination. How are they populating the origin and destination fields on small Part 91 IFR flights being conducted by a Cessna 172? 

Do they have some sort of access to the Aeronautical Message Handling System (AMHS) or Aeronautical Fixed Telecommunications Network (AFTN). If not, how else could they be receiving this data?

In reference to the possible duplicate question, it's unclear to me if the term "flight data" in the context of that thread is in reference to the near real-time (NRT) flight track flight data, or a more robust data set which includes the details of a flight's route,  as filed and/or cleared.

Normally that type of flight plan routing data is transmitted over the AFTN/AMHS. It would be very very interesting if the FAA is integrating AMHS flight plan FAA data into the ADS-B/SSR flight track data. 

Whilst I doubt know for sure, I’m relatively certain the flight plan routing data is integrated with track data by air traffic management (ATM) system and available to ATC personnel. What I need a better understanding of is how and at what point that integration occurs. 

Is it an automated process occurring upstream or is it entered manually by the ATC personnel? If it's automated then is it AFTN/AMHS data or another source? If FlightAware is receiving this data, is it included as a single feed our do they have a separate feed? If the flight is scheduled, assuming the majority of origin/destination information results from callsign/ flight schedule cross-referencing, if either sources routing information conflicts with the other, which has primacy? 

I concur that I understand the 5 minute FAA track feed delay was eliminated, but I don’t have a source to confirm.
The data that FlightAware first receives for most commercial flights
  is sometimes delivered a year in advance via schedules published by
  the airlines. That data then usually remains unchanged until a few
  hours before the flight. Pilots or air traffic control operators
  controlling airspace on the route of flight will push out new data
  with a flight plan that provides specific details about the planned
  routing, altitude, and speed of the plane. But that’s where the static
  data end and the real-time data begin.
  
  “When the door is shut and the parking brake is released by the
  pilots, the aircraft or airline will send us an ‘out’ message that
  indicates it’s pushed back from the gate, and we know that departure
  is imminent,” says FlightAware’s Baker. “As soon as the weight is
  lifted off the landing gear, we often get an ‘off’ message from the
  airlines or the aircraft itself that indicates the plane is airborne
  or a departure notice from air traffic control.”
  
  Once en route, FlightAware continues to get position updates from the
  plane via radar installations at air traffic control centers and via
  ADS-B, along with continued updates on the plane’s route from air
  traffic control. All of this information is then fused together by
  FlightAware’s software, which uses it to determine the plane’s
  estimated time of arrival, and then displays the aggregate data for
  the flight on its website and in its apps.
  
  “We detect the landing either by seeing the plane slow down below
  flying speed, by getting an ‘on’ message from the airplane, or by
  receiving an arrival notice from air traffic control. From there, we
  may have surface coverage to track the taxiing on the ground, and
  eventually receive the ‘in’ message from the airline indicating it’s
  safely parked at the gate,” says Baker.
  
  Now multiply all the above steps by the thousands, or one for each
  plane in the sky across the world at any given time, and you’ll begin
  to appreciate the amount of data that fight tracking websites’ servers
  and algorithms are crunching each and every second.
  
  https://www.fastcompany.com/3044490/how-flight-tracking-apps-work