What is the cause of high pressure in the earth's core?

Some would say gravity, but since the center of the core is effectively null gravity due to all of the mass of the earth located equidistant from it in all directions, then I think the pressure at the core is considerably lower than others estimate. probably the highest pressures would not be at the core, but at some point between the core and the mantle, some of this depends on a liquid core or a solid core model...how can a solid be pressurized internally if it is free-floating at the center of the mass of a planet? You know, they tell us that the Earth's core is molten iron and nickel and that it is a liquid core since the Earth has a strong magnetic field....but don't you remember learning that heat destroys magnetism? And look at the magnetic field lines in the arching solar flares from the sun itself! How can these be magnetic if heat destroys magnetism....why is the sun magnetic if it is made primarily of hydrogen and helium? One scientific question always seems to lead to others...seems like a fractal progression....