Spoilers and a bit of text:
There are many kinds of monsters. That's why Seiichi also asks this question to Koko or Miroku. Since some are more similar to humans in terms of reasoning, thinking, and so on. But others, on the other hand, are more wild or unstable, like the mantis, for example. Perhaps they do not have the same notion of good and evil, or even decide to abandon the idea of integrating into human civilization.
In Koko's case, she is more stable and human-like, so she can wear clothes and be practically like a normal person. But another monster of a different nature and form, perhaps even if it tries, may eventually succumb to its instinct and never be able to integrate 100%. That is why there are areas in the cities where they live more separated and in a more controlled way.
The monsters that are totally friendly and functional, live directly with humans despite the occasional instinctive rejection of some people (just as many monsters repudiate them). Those that don't, descend into the interior of such non-human zones. As I believe is also mentioned, the authorities may offer criminals as food, or directly, to fight each other without disturbing the humans on the other side.
In my last visual novel I mention a bit about that at the beginning as well. However, these are aspects that I haven't fully concretized or gone into too much depth yet. But maybe I will do it in the next one, because at the moment, I'm making the first sketches about a possible sequel.
If there is curiosity about that, I could try to explain more about the world or give more background/stories and that sort of thing. In a future visual novel or something.
Thanks for your comment and curiosity!