Shell theorem is a simplification stating that a spherical body affects external objects gravitationally as though all of its mass were concentrated at a point at its center.
The direction of Earth’s gravity toward the center of the Earth made some flat-Earthers to assume that we think gravity is caused by something in the center, and that our inability to go there “disproves” gravity. They are wrong. Earth’s gravity is the result of the entire mass of Earth. Using the center of the Earth is merely a simplification.