Are airliner flight plans/routes preset?
Airlines have an own department that is solely responsible for their flights and flight plans: Dispatch. Since single airways can have different availability or just direction for different days or even times of the day, there will be a preset pool of routes for recurring flights, which can then be adapted for anything out of the ordinary, like NOTAMS, weather forecasts, operational requirements like airspace closure, etc...

The flight plan is usually submitted by the dispatchers and not the aircrew.