Hmm, how strange - in the first two videos the horizon and the ground below are covered with clouds - entirely or partly.
To see 'the curvature', one must look exactly at the ground below.
I wish I could share photos I once took from the airplane window. The sky was clear and I swear, there was a curve!
Yes, you did indeed see a curve and your photograph does indeed show a curve. The window in the airplane is curved to fit the body and this creates a convex lens that slightly distorts the image seen.