They surely get data from other sources too. You can track planes flying across the ocean, where they get that data from?
https://www.flightradar24.com/blog/the-evolution-of-flightra...
https://www.flightradar24.com/blog/over-the-ocean-24-hours-o...
https://www.flightradar24.com/blog/tracking-flights-with-sat...