Question

How is a Potato stem it should be a a root

Answer 1

The potatoes are considered stems because the potatoes grow on underground stems, called stolons.
You can know potato tubers are actually thickened stems because they have buds that sprout stems and leaves. Roots don't do that.
As i said earlier, potato have buds on it, which under the right conditions can lead to the growth of a new plant. Actually the presence of these buds make it a stem, because roots do not havr buds from which new plants can come.