These cells eventually split into two, and then these two split into two again, and so on, growing and forming various parts of the body. In the case of a human being, one cell grows into a baby weighing about 3 kg in about 9 months, and then the baby comes out of the mother's tummy. After that, the body continues to grow until it stops growing after about 20 years and becomes a complete human being.
It is thought that the reason we grow in this way is because we have a growth program written into our genes. Everyone has genes, but they are so small that they cannot be seen even with an electron microscope. Our bodies live by deciphering the programs in these tiny genes.
The mechanisms of living organisms are more complex and precise than any machine, and are truly mysterious.

The fertilized eggs of all mammals are almost the same size. Elephant fertilized eggs are large and mouse fertilized eggs are small. As for why elephants grow large and rodents do not, this is still a mystery, except that it seems to have something to do with genes.