I unfortunately fell down a flat earth worm hole a while back so have some info on their thought. Planes don't go high enough to see is the argument. Radar is rigged to make planes fly in circles to make the earth appear as a ball. Also the fact that planes fly with their noses up is proof that the earth is flat. If the earth were a ball like the "lies" that are spread, planes would fly with their noses pointed down, otherwise, they would fly right off the earth. Yes, that is some of the "logic" they spew.
Basically after getting in deep enough of the youtube video, and you follow the dots, it comes down to a creationist argument. If the earth is flat, it doesn't rotate around the sun (the sun and the planets rotate around the top of the flat earth), and the earth can once again become the centre of the universe,.