I will start off by saying that personally I don't have any investment in the shape of earth. I am watching the first video. The initial argument made by the narrator is that when ameture rockets and balloons are sent up to 20 miles high they cannot see the curve.

Assuming it is true that the radius to equator is just under 3000 miles a balloon at 20 miles altitude would only be .5% away from the surface of the earth.

If you had a table and a tiny camera hovering .5% off of that table you would also not be able to determine the size or shape of the table.

His following arguments seem to throw out the concept of gravity (saying water flows downhill and so the ocean wouldn't be flat), which makes me curious how flat earthers explain gravity.