Inside the event horizon of a black hole, our current understanding of physics breaks down, and it becomes exceedingly difficult to describe or predict what happens. The event horizon is the boundary surrounding a black hole beyond which nothing, not even light, can escape its gravitational pull.
According to general relativity, which is the current theory of gravity, as an object crosses the event horizon, it experiences extreme tidal forces, where the gravitational force becomes significantly stronger on one side compared to the other. These tidal forces become infinitely strong at the singularity, a point of infinite density at the center of a black hole. However, it is important to note that the singularity is a mathematical concept, and our current theories do not adequately describe the physics at such extreme conditions.
From an outside observer's perspective, time appears to slow down as an object approaches the event horizon due to gravitational time dilation. This effect means that from an outside perspective, an object never actually crosses the event horizon but instead becomes increasingly redshifted and appears to freeze near the event horizon.
As for what happens to an object or matter once it crosses the event horizon, our understanding is limited. The information about the object's internal state is seemingly lost, which is known as the "information loss paradox." This paradox is an active area of research and a subject of much debate in the physics community.
It is important to note that our understanding of black holes is based on current theories, and future scientific advancements may refine or challenge our understanding of what occurs inside the event horizon.