This is a labelled toy model, not a physics simulation. The numbers come from one honest gravity rule (pull = mass divided by distance-squared), orbits follow Kepler's rule, and the boundary is marked where the simple escape-speed estimate reaches the speed of light. That Newtonian estimate happens to give the same number as the full theory of general relativity, but our boundary drawing is a schematic diagram - nothing inside it is computed or shown as physics.
What is real: from far away, a black hole's pull is exactly the pull of any object with the same mass; black holes do not suck things in; the event horizon is a point of no return, not a surface; and we find black holes by their effects - stars orbiting something invisible, glowing gas, bent light. Astronomers really did weigh the black hole at the center of the Milky Way by watching stars loop around it (2020 Nobel Prize). What matter is like inside the horizon is not known.
Left out on purpose: spinning black holes, wormholes, time stretching, and anything inside the horizon. No real observations, locations, or personal details are used; nothing about you leaves this page.
Sources: NASA Science's Black Holes pages and ESA Kids' Black Holes pages. Links are in the project readme.