I’m assuming there is more to it than just the radio data? In FR you can see planes everywhere, including thousands of miles off shore.

My guess is that FR24 just looks up the flight path from the flight number, and then once an airplane goes “off the radar” it just assumes it goes the shortest/great circle path towards its destination airport - at which point it will again be in a “known” spot? If this is how it’s done then one should see lots of planes approaching e.g the US east coast suddenly “shift” from their estimate position to their transponder positions, as they are picked up by receivers.