Answer: A person’s soul grows from scratch just like an embryo from a drop of semen develops in our world. Instead, the embryo repeats the same stages of development as the soul.
Once in the right place, the semen, or the so-called Reshimo, that is, a given spiritual state begins to develop. Conception occurs in the world of Atzilut, and then the further growth of the soul continues.
A person, of course, does not know how his soul develops. He begins to feel this when he goes through the stage of spiritual birth and then the stage of external development, which is carried out not in our world, but in the attainment of the upper world.
From KabTV’s “Fundamentals of Kabbalah,” 2/16/20