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.
No I already read Sweet Home before. What I wanted to know is WHERE does Zero and the Variants really come from? They can't procreate and just basically appear out of nowhere. One and Zero are seen talking to the "Earth". Just want to dwell into that more.
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?