I recommend reading sweet home (shot gun boy is the prequel to it) to get an answer to your question :))
SPOILER IF YOU'RE LAZY:
Basically in sweet home, the people turn into monsters if they have a very strong desire and died without fulfilling that desire. For example, if someone wants to be the fastest runner and is obsessed with the idea of being the fastest runner but dies before fulfilling that, they will turn into a monster. And usually, the monster form they have is something to do with their desire, so in my example, that person might have super strong legs in monster form.
Maybe it started as an alien or some kind of alternate life form that came to Earth? It could also be some kind of mutation based on the degree of evil a diseased human posesses, that might turn them into a monster, like what happened with Seongbin. A single being that powerful would easily be able to turn others into similar creatures, like Zero did to Gyuhwan, who later awakened supernatural abilities :p
I was also kinda confused by Zero and One talking to the Earth, but I'm guessing it was some kind of illusion/imagery they created as a metaphor to represent what the purpose of their powers were? For example, zero saw Earth as something that he could fix by killing humanity, whereas One choose to use his ability to wipe out 70% of all ppl, leaving only a select few that meet his standards
But all that is just speculation, from what I personally interpreted, only the author would rlly knoww (T^T)
Maybe the variants started as an alien or some kind of alternate life form that came to Earth? It could also be some kind of mutation based on the degree of evil a diseased human posesses, that might turn them into a monster, like what happened with Seongbin. A single being that powerful would easily be able to turn others into similar creatures, like Zero did to Gyuhwan, who later awakened supernatural abilities :p
I was also kinda confused by Zero and One talking to the Earth, but I'm guessing it was some kind of illusion/imagery they created as a metaphor to represent what the purpose of their powers were? For example, zero saw Earth as something that he could fix by killing humanity, whereas One choose to use his ability to wipe out 70% of all ppl, leaving only a select few that meet his standards
But all that is just speculation, from what I personally interpreted, only the author would rlly knoww (T^T)

I wished Zero survived and not absorbed by that Muzan wannabe. I loved the troupe where monsters learn how to be human.
Also did they explain where Zero and the monsters really come from? Is it just a power trip of some gods?