>>20267252They use the red line?
I don't see your problem then, it makes perfect sense, it's a straight line.
It would be the same on a globe though. The northern circle is the same on flat earth and ball earth. It's the outer circle (the supposed southern hemisphere) that makes flight routes unreasonable on a globe. No nothern flight has to cross the equator ever. But some southern flights from one southern location to another southern location do need to cross the equator, that's the weird part on a globe. But it makes sense on a flat earth.
Here is how that looks like:
https://youtu.be/IivYbbtoCIE 