What differentiates a supermassive black hole from a stellar black hole?

Their size

Supermassive and stellar black holes are primarily distinguished by their immense size. While a stellar black hole is born from the collapse of a single star and generally has a mass up to a few tens of times that of the Sun, a supermassive black hole can contain millions, even billions of times the mass of the Sun. These giants are often found at the center of galaxies, including our own, the Milky Way. The discovery of these imagination-defying objects has not only confirmed predictions of Einstein's general theory of relativity but also continues to pose fundamental questions about the formation and evolution of the universe.