The earth is an oblate sphere with slightly flattened poles. The poles of Jupiter and Saturn look flatter because their atmospheres are very dense.
The reason why stars, planets and other celestial bodies are all spherical, not square or other strange shapes, is entirely the result of gravity.
Any object will attract others. According to Newton's law, the magnitude of gravity is inversely proportional to the square of the distance between two objects, regardless of the position of the objects. Therefore, a limited number of unevenly distributed and identical particles always tend to gather together to form spherical clusters. There are many other forces in the formation of planets and stars.
Suppose that after a period of BIGBANG, a large number of different particles are unevenly distributed in space, thus forming a large cloud of unevenly distributed matter. In this material cloud, particles attract each other, but the overall gravity is not balanced, so there are still some disturbing forces to make it rotate. Some particles may get a companion star, so there is gravitational interaction between the two celestial bodies. Of course, it also involves complex problems such as electromagnetism, tribology and heat.
At this time, the scattered material clouds gradually gather together under the action of gravity, and at the same time, due to their own inhomogeneity and some external forces, they begin to rotate, thus forming a roughly (not completely spherical) rotating celestial body. Its shape will depend on its rotation speed. The faster the rotation speed, the closer its shape is to an oblate circle. In addition, the shape of this celestial body is also related to the density of its constituent substances.
Suppose there is a standard spherical billiard ball, which will keep its shape approximately spherical when it rotates; But if it is a rotating water-filled balloon, it is a flat sphere with flat ends and a protruding middle. In fact, most celestial bodies have great mass and high rotation speed, so the material near the equator is likely to be thrown away from the celestial body and given a "slimming movement". Dumped "fat" may be scattered around, and in some cases, it may form a spherical satellite through a similar process.