Loki and Sylvie, the Variants in love, is a beautifully multilayered metaphor and an interesting concept.
Here I want to try analyzing this metaphor in more detailed way.
The existence of the alternative versions of oneself is a SciFi concept that is used quite often in such Fiction, but it is mostly explained differently in the different canons.
I would say, the MCU laws for being the alternative universe versions of one being are somewhat different from what we have in the other SciFi examples.
In MCU, the Variants do not necessarily share the same DnA, genetics, they can be different on the molecular level and have the different experiences. So, what exactly connects the Variants of the same being? The answer for this is more Fantasy.esque in its nature than SciFi.ish: it is the Soul.
The Variants share the same soul, they are the same in their essence, their sameness matters in more... Metaphysical terms and realm.
So, basically, Sylki is the most perfect example of the Twin Flames & Soulmates trope.
They, the two completely different, unrelated people, look at each other and see the reflection of themselves in one another.
By loving Sylvie, Loki learns to both love himself and the another. You know how they say, one has to first learn to love oneself before they learn to love somebody else? Loki kind of kills two birds with the one stone by loving Sylvie. He also sees her, the Heroic Sylvie, and starts to understand that maybe there is more to his essence, to his soul, than being the Villain.
Sylvie, on the other hand, found in Loki something she considered as an impossible. A person whom she can trust, someone who selflessly loves her for exactly who she is. In him, someone who shares her soul, thus the essence of her being, she saw that maybe there is more to her life than fulfilling her glorious purpose.
So, yeah. Sylki's story has to be one of the most beautiful Mythology inspired SciFi love stories out there.