The mass of a black hole is concentrated at its singularity, which is a point of infinite density at the center of the black hole. The singularity is a region where the laws of physics, as currently understood, break down. It is a point of extreme gravitational force and is surrounded by the event horizon, which is the boundary beyond which nothing can escape the gravitational pull of the black hole.
The event horizon itself does not contain the mass of the black hole. It is simply the point of no return, where the gravitational pull becomes so strong that even light cannot escape. The mass of a black hole is considered to be located at its singularity.
While black holes have a significant gravitational effect on their surroundings, it is important to note that this gravitational force is a result of the mass at the singularity. The singularity represents an extremely dense and compact region where the mass is concentrated, and it is the source of the gravitational field associated with the black hole.