Your soul is not your whole conciousness that is also made up of your brain, natal chart, lived experiences and of course your choices in this life, as for how a soul enters in "parts" frankly outside of HPHC and Maxine its unlikely any human understands this process and even with those two its unlikely they fully grasp it, reincarnation is a Godly science we dont have words to explain the base concepts at play but in essence I believe the initial "parts" may refer more to the energies of our solar system creating an energetic foundation e.g the solar and mars energies of ones soul and its likely your conciousness finds its seat in the soul afterwards but this is just my assumption, doing Anbuis's ritual and asking him for knowledge on the matter would be your best bet to start learning.
We know the soul can be split into multiple parts as the Gods do this, it stands to reason that it can be reincarnated in parts also, the fact HPHC said abortion is morally acceptable indicates that abortion during this process has no long term negative ramifications for the soul and in fact this may be why it happens in stages specificly to protect us from having our souls harmed by events such as miscarrige or abortion but yet again this is just an assumption.
The truth is the occult is a science, but to most of us its simply magic because we dont understand it yet, the actual mechanics at play are beyond the understanding of anything but the most advanced occultist.