Basically, on the physical level, we have the mind and the body as two different things. The things are interlinked. The mind, for the most part, controls the body. It thinks, and it controls actions, and the body does those actions. But it's not entirely dominant over the body; it does not have full dominion over the body. The body is responsible for many physical things, which can have an influence over the mind.
Exhaustion for instance is an easy thing: if the body is tired, doesn't matter how much the mind is awake the mind will cave in eventually. Emotions are dual-level. Some emotions come from the mind and are then felt from the body, but other emotions come from the body and overwhelm the mind. In fact, emotions are almost something outside of both.
But basically. The point I'm making here is. The Mind Is (Mostly) Not A Plaything Of The Body; the body is mostly controlled by the mind but the mind can be influenced both positively and negatively by the body. For instance, a female mind in a male body often leads to dysphoria when the body gives distinctly-male-anatomy urges.
The link between mind and body which also holds emotions is the spirit. The spirit is essentially the sum of the parts and then some when it comes to mind and body. The spirit self is our "true self" as it were. Our spirit selves can change over time as our perspectives of ourselves are altered. For instance, someone who is 20 years old probably doesn't have a spirit who looks like an 80-year-old; someone who is 80 years old might have a spirit that's 20 years old in appearance but is more likely some other appearance closer to their current form.
Our spirit selves always have the appearance of our mind's eye. A pre-transition transwoman isn't male in spirit, but rather, female. And appears that way more likely than not. Our spirit selves often are going to be relatively close to our own appearance more likely than not.
For instance, it could theoretically happen that a Caucasian blue-eyed short-haired blonde would have a spirit self that's dark-skinned, orange-eyed purple dreadlocks...but it's not very likely. More likely is simply a look close to what they see in their every-day body.
To put it another way. The spirit is essentially the heart and core essence of a person. It is what we are, in the current moment. It is shaped both by our soul (our core personality which could never ever possibly change) and by the real world (our body/mind), in that it is a link between the two, shifting as we see things.
I'm not quite sure if I'm describing these well enough (I feel like there's so much I'm leaving out), but that takes me to the final level: the soul. What is the soul, if not the heart of who we are? The soul is the heart of everyone we could ever be. In other words, whereas the spirit is "us in the current moment", the soul is "us in every possible moment". Because souls transcend space and time, existing outside those boundaries to me.
Souls represent us at the fundamental level, linking us to our spirits across any and every universe, existing in a form transcending what we'd know of being consciousness. Souls don't so much think or feel, as much as they do experience and then share. Souls share with other souls, and blend together.
This is, loosely, where soulmates begin to form for me. Because souls that begin to blend together tend to be more compatible with one another. And because souls transcend space and time, it can even be possible for multiple people living at the same time to have part of a merged soul. In short. There are multiple people (not just a singular one) we can be compatible with on the fundamental level.
This is also why people who're soulmates can have relationships not last, too. Their souls might indeed be mated together, but that doesn't mean that in our universe at that particular time their vessels, their spirits, will experience the link between. So we can in fact have more than one soulmate, because souls tend to be a bit...flexible.
Now to be honest.
I'm not all too familiar with the anatomy of a soul.
This is all just my belief anyway, sure, yeah, but even for it just being my own belief, most of this is just speculation. I don't know the details. I don't know if I'm just making up stuff or if this belief in the system I'm describing is coming truly from me. But I hold it all the same.
Essentially in summary, souls exist in a form outside of our universe. The link to those souls is our spirits, which exist in our current universe. Our spirits are intangible and ever-changing, ever-evolving. Our mind and body are the physical vessels in which our spirits reside.
Our spirits also hold some traits which will remain true regardless of our mind/body and can even override them, e.g. a female spirit causes a female mind even if the body is male. But they're not immune to influence since a spirit may change appearance off of experiences from the mind and body. The soul is essentially the archive of the spirit as it were, both documenting and dictating much of what the spirit is, has been, and will be. And this archive seeks out other archives to cross-reference as it were.
This probably makes no sense.
Not all love is between soulmates nor are soulmates going to necessarily always have love.
Yet the two have obviously a very huge overlap, in that--yes. Those who're soulmates with one another will find each other, will have love for one another, and will be able to make it work because they are connected on a deep, fundamental level and they know it. <3