I prefer using ForeFlight with Lockheed Martin. I have Verizon cellular and can brief, file, amend, activate, close, or cancel from the cockpit.
There are many providers of flightplan services, not all are free. DUATS providers contract with the FAA, but others may send flightplans directly to ATC or in the case of VFR flightplans, they send them to the FSS which in the CONUS is operated by Lockheed Martin. In Alaska, the FAA operates the FSS. Only sufficient information is sent to the relevant FSS to be able to determine if a flight is overdue and to be able to activate and close a flightplan. If a SAR operation is required, or route information is needed, the FSS facility will retrieve the route and other data from the outfit that holds the flightplan details. For IFR flightplans, they are routed to the appropriate Center, but again much of the information is not transmitted and if needed must be retrieved by ATC from the outfit that initiated the flightplan and holds the detail data. An example is the alternate airport is not transmitted to the ATC computer, so the controller does not have a clue what it is.