When a ball is thrown vertically upwards and then lands in same position is the displacement 0

Displacement is the distance between the initial and final positions of a moving body,unlike the distance which is the length of the actual path traversed. Hence,if a body returns to its original position after completing its motion,the displacement must be 0. (This is what happens in case of a body thrown upwards or one moving in a circular path.)
Hope this helps :)

that is correct; displacement measures the difference between the starting and ending points

this also means that the average velocity for this trip was zero, since average velocity measures the average rate of change of displacement