There are many non-stop long-haul flight routes between two locations in the southern hemisphere, all without crossing the northern hemisphere. These routes can only exist if Earth is a sphere.
Some flat-Earthers claim there is no non-stop long-haul flights between two locations in the southern hemisphere. Flights need to make a stop in a location in the northern hemisphere because the distance will be too far and such flight will traverse the northern hemisphere, or more accurately, the center of the Earth according to their model. They are wrong. There are many such flight routes.