The simple explanation
Humans aren't born singular. Nearly all models since the dawn of psychology agree that a child is complex and very compartmentalized. They're born plural or set up for plurality anyway. According to psychology, according to childhood development models since before Freud. Children are a bunch of separate self-states, sensory-states, ego-states, etc.
Plurality in adulthood occurs when something encourages remaining separated -or- when there's a lack of role-modeling and encouragement for a child to become singular.
The singular-centric models of childhood development say that children "naturally" fuse over time that these separate states or egos etc. eventually merge and blend, but they don't explain or show or (of course!) don't experiment on to prove this is the "natural way of things." It's assumed. It's magical. It "just happens that way naturally."
Much more likely would be that it happens via the natural bonding & mirroring instincts: "peer pressure" and conformity, mirroring role models, etc. that children fuse over time to be one. Basically, they let go of or spontaneously merge the "others" because it's not accepted in society. With positive role-modeling of singularity and other ethics and morals, the child probably lets go of or merges and suppresses the more unacceptable "self-states" and becomes more like their role models over time.
It's pretty doubtful that this is a magical process that just happens and since children are born to adapt to almost any circumstances (tribal culture through Star Trek), if a child is born into a plural society with acceptable plural behaviors to model themselves after, we would presume that a child would be naturally plural for their whole life and there would be no culling or suppressing of other self-states except of those self-states that do not fit into their surrounding culture or meet their role model's expectations. Such a child could grow up with a high level of co-consciousness being relatively trauma-free and with positive role-modeling around having a good internal community and culture, and the sharing of internal information, memories, and senses.
(In other words, it's very doubtful that singularity "just happens naturally" as some models appear to claim like the self-states are magnetized to each other and fore-ordained to merge by their very nature even if a child grows up alone in the world. It is far more likely that singularity is predominant and impressionable children model themselves after the predominant culture.)
In any case, if every model shows that there's this separateness in infants, then the separatenesses all growing up separately makes them a group entity. They grew up "in parallel" at the same time. Simply by a lack of merging, or by following plural role-models rather than singular role-models, or because their singular role-models didn't exert that singularity in a way that shapes or molds the child's internal structure (i.e. allows a child to have 'imaginary playmates' and never discourages them from talking amongst themselves, does not condemn contrary behaviors by enforcing consistency, etc.), or the child's environment encouraged remaining separated in some way (playing "by oneself" as an "only child").
Whether or not a plural was traumatized during childhood development or not, according to psychology we all started out plural. This means we are all equally born to the body, and equally have rights to a shared life. There is no "host" or "core" with the potential exception of those who had started to fuse, merge, and cull self-states and were becoming a singular and then regressed back to plurality due to events that shook the foundations of their newly emerging singular identity.
A little more technical
As psychology matures, and the understanding of the human mind is expanded, more and more models of typical human development emerge.
It is generally believed that the infant mind is a series of impressions and state-dependencies: being hungry, feeling pain, needing attention, being scared, being fed, being nurtured, being uncomfortable, etc. A human does not form a mature and integrated personality until later in childhood or even in the teen years. Some models even suggest that humans do not gain full agency and self-awareness until their twenties.
When these states are validated, and when caregivers react appropriately to these states being fed when hungry, being comforted during pain, being interacted with and touched when lonely, protected when frightened, etc. a child is given a secure basis to explore personhood and develop ideas, thoughts, give voice to needs and desires, develop preferences, etc. and bring all of these varied states as they change and mature with development into one person-ality.
When the milestones of life are interrupted in any way, from the womb and onwards, with neglect of these needs, with additional threats from caregivers, with physical, mental and emotional violations, with erratic caregiver behaviors, even with floods of anxiety chemicals in the womb itself the child's psyche may not emerge with a single sense of personhood or one person-ality. Each state is left to fend for itself as it were, a trial-by-fire of development of thought, strategy, ideas, and adaptability but shuffled amongst the varied folds of the states. Additional states may be induced in such a tumultuous environment that would not have been needed in a properly nurturing environment: the need to play up to abusers, the need to lie, the need to protect oneself, the need not to cry, etc. Some states may get stuck at various stages of development, unable or unwilling to progress later becoming Littles.
Meanwhile, the struggle for survival continues hampered, unsupported or improperly supported by caregivers and the environment. Humans are survivors if nothing else. The child unwittingly allows (indeed needs to allow) their varied states to adapt to an unpredictable, volatile, unstable, lonely, or threatening situation becoming hyper-vigilant to cues from the environment and other people, while being highly malleable in response to external events and situations.
Multiples may experience this disruption of development exceptionally early in childhood development, or even before being born through anxiety chemicals, drugs, sounds or even physical battery of their mother while in the womb. When experienced "early enough" in development (which can vary from person to person), there isn't even a hint of the "one person" they were potentially to become had the trauma not occurred. The trauma was experienced so young there was never any one person-ality emerging in the first place.
Given this model, one could say we're all born multiple, but when normal development is interrupted we remain multiple and all these various states remain segregated to optimize our chances of survival in a hostile developmental environment, or in an environment where we are best off relying solely on oneself.
Even then, experiencing trauma at a very young age or in the womb does not predispose a person to definitely becoming multiple. There's still the potentially genetic predisposition to dissociation to consider which allows for more compartmentalization of both physical impressions and memories and if there are stable cornerstones of a baby or child's life, they still may develop one or more stronger person-alities that appear to be essentially well-adjusted or "normal".
Other lenses or explanations of being "born multiple"
Some multiples have had the experience of others sharing the body with them since birth, whether or not they were traumatized in any way. (Some remember being multiple before they were abused, or being abused or punished because they were multiple.)
Many such multiples feel that plurality is simply a natural variant of the human mind-- something which, like sexual orientation or gender identity, is either inborn, genetic, or hardwired in at such an early age as to make changing it later in life impossible.
Others have offered spiritual explanations for their multiplicity. Some see multiplicity as the result of persons from past lives sharing one's current body. Others see it as the result of having channeled someone in before birth.
Other models constructed with contributions from Shiu of Amorpha.