There's a flat earth map that people post around, basically the picture where they take the actual map and the north pole is in the center, and south pole is surrounding everything with the "ice wall." If you just apply basic geometric principles to that map you can see that it's completely wrong. Australia would need to be more than twice the size it is because it's so far away from the center.
This isn't even getting into the geometry of the sun relative to places on the earth. Just get some of your flat earther friends and go to different points on the earth at different latitudes and measure where the sun is at the same time. You can just plainly see the earth can't be flat. As a bonus, if you use your measurements you can actually measure the curvature of the earth too!
Not only that but their sun and moon paths would never be able to produce the day/night cycles that we have. They now claim they are projections but they never explain why or how these projections exist, let alone their paths to produce light only in a few places. Let's not talk about the million pictures of the earth or countless videos and livestreams and the fact that they have 0 photos of the flat earth, the dome, the Antarctic wall that surrounds the earth? How difficult is it, I mean if we can have hundreds of thousands of fake pictures, surely they can have at least 1 ''real'' picture of the flat earth, no?