Q.
The earth is an approximate sphere. If the interior contained matter which is not of the same density everywhere, then on the surface of the earth, the acceleration due to gravity
There's no such thing as zero gravity. Weightlessness and zero gravity are two different things. The earth's gravity keeps the moon in orbit. And astronauts are generally much closer to earth than the moon is, which means that the earth's pull on them has to be much stronger.