OG world

The real world is the ABO world.
Their first life is what Heemin considers the "novel" world, but since everything was traumatic for him he creates the non-ABO world to cope. Hun somehow ends up there with all his memories and tries to be a better person to Heemin who doesn't remember their first lifetime/real world. Then they end up back in the real/ABO world, but back in time to when things first started going bad between Heemin and Hun. Heemin thinks the non-ABO world is the real world and the real world is the "novel world". Hun eventually remembers everything, both their first life in the real world and in Heemin's non-ABO world but doesn't want Heemin to remember their first life. They're both trying to make the current timeline/life better. Hence why things are different and certain plot points are either coming up sooner/being resolved quicker.