How does FlightAware correlate flights to Part 91 flight plan data?
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