OK. If I remember correctly from the beginning of the one-shot, there were 5 things that lead to a culling of human population from at least 7.4 billion (if the first of those events is a few decades into the future, probably more) to 2: the bright comet hitting Earth; the virus on that comet causing a zombie apocalypse; the human defensive system using nukes to end the zombies; aliens coming to raid, loot, and pillage; a cataclysmic natural catastrophe occuring. All of these culled some part of the human population, and I'm certain that, after the first 3 events (comet, zombies, nukes) a large, I'd even dare say overwhelming majority of the human population already dissappeared. The later landscape of Earth resembles your stereotypical post-apocalypse setting: life exists, but pretty much reduced, a shell of its former self, and there is also a lot of radiation due to fallout.
This period could've also seen massive population movements, most likely as a result of people trying to find new lands to settle in where there was less or no radiation (kinda reminds me of Silesia in season 2 of Algerian Mapper's Alternate Future of Europe, wherein their population doesn't even know where they came from, but they know it was far to the east or northeast, from where they migrated to avoid the radiation, came to Silesia and took the local identity, name and symbols). This may be how Nika's family (almost certainly before she was born) came East (I suspect somewhere in Primorye or Sakhalin or Kamchatka).
Then, the aliens arrived. However, given what was told at the beginning of the one-shot, I'd say they didn't come to Earth as invaders or conquerors, but rather as the spacefareing equivalent to piratical sea kings or nomadic horse raiders. I imagine that, as technologically superior as they were, and as much as humanity might've been culled at that point, these alien raiders were probably still heavily outnumbered by the remaining human population. I'm willing to assume these aliens just went around Earth, raiding, looting, pillaging and sacking along the way, on occasion fighting human armies, and, more often than not, winning.
Yet, the radiation and fallout were an ever-present threat to both sides. The aliens, technologically advanced as the are, could've had a large amount of radiation absolvent materials in case of finding an irrediated world. They could've used those materials to clean the Earth from fallout in order to make raids on Earth less risky. If this is what happened, it would also have a beneficial effect on humanity, as less radiation and fallout meant less cancer, meaning more healthy humans, meaning a bigger growth in human population, meaning more humans, meaning a larger labour pool, meaning larger production output by humans, meaning more things for the aliens to loot.
In this confusing period, nations may have made some kind of treaties of common defense against alien raids, which may explain both why Nika was consripted into the Russian Armed Forcey, and how did she end up in Japan. It was probably during one of those alien raids in Japan that Sayu and Nika first met. Little did they know that, as fate would have it, they'd become the only humans to survive the culling.
Which leads us to the last of the events in the culling: the cataclysmic natural disaster. Honestly, it remains unclear whether Sayu and Nika fell in love with each other before or after that cataclysm, but, in either case, the cataclysm did kill every human except the two of them, and many of the aliens died due to their spaceships getting shipwrecked as a result of the cataclysm. Whatever aliens may have survived this were probably extremely shaken by what they've just witnessed. They may or may not have tried searching for survivors or resources in the wreckage, but whether they did try that or not is irrelevant, as they just left afterwards.
This then leaves us with the events of this one-shot, importantly Sayu and Nika finding one of the shipwrecked UFOs. There they found a system which allows for same-sex reproduction. Who knows, maybe those aliens were advanced enough that this sytem included several layers, including one that negates any risk of genetic defects. If that's the case, it may be completely fine for their offspring to procreate with each other.
I could go on with how much we can predict different scenarios for what would happen with their descendants, but that would require a lot of time that I don't have now and a lot of mental gymnastics I don't feel like doing at the moment, but suffice to say that one could make several series (maybe even a mapping one!) from that.
