I am trying to find out in layman's terms how does ATC determine the direction a flight has to take after takeoff, or when should it start descending for the destination airport. I understand it has to do with the flight plan but Googling revealed lot of technical stuff that I couldn't understand. In my limited experience of listening to LiveATC and YouTube video, ATC never inquired about the destination - it felt like they already knew.
Is destination information always available on the screen to ATC or does the flight crew request changes based on their flight plan? How does it work when you cross international boundaries? Is the same flight plan available to ATC in all the countries the flight will be flying through?
For example, a flight taking off from SFO could be northbound towards Seattle or southbound to LA or eastbound. ATC has to make room for the proper direction of the flight and hence needs to know the destination.
To complicate matters, I have seen flights taking different routes while the destination is same e.g. one SFO-FRA flight hugged the west coast (magnificent views of the coastline) up to Canada before making a right turn, whereas other times - but with the same flight number - it kind of crosses the continental United States diagonally. So I feel it's not just about the destination, there are other factors involved.