Newton's law of gravity says that the greater the mass of an object, the greater the gravity. Why does the greater the mass of an object, the greater the gravity? How is gravity generated?

According to the general theory of relativity, the distribution of matter in space will affect the existing form of space-time. In short, mass will distort space-time, and three-dimensional space will bend in the fourth dimension due to mass, forming geometric constraints, just like the concave edge around matter, which will cause the surrounding matter to "sink in". The greater the mass, the greater the bending, which is equivalent to gravity; Classical mechanics is not introduced here, after all, it has been considered as a special case of low-speed relativity; I don't understand quantum mechanics, but it really can't describe gravity; There is also the view of modern particle physics that gravity is produced by the reciprocating motion of gravitons between substances, and I don't know the details.