There Was No Secret Evil-Fighting Organization (srsly?!), So I Made One MYSELF! There is no better world build than a real one.

Didn’t I say to make my abilities average / So I’m a spider so what?

What more believable than a world that has been destroyed by humanity multiple times?

Since I'm sick and spending most of my time in bed I had a while now to think about this and I'm sitting at the moment on the conclusion that there are two answers to this. One one hand there is obvious stuff like Bookworm or Wild Game that spent just an huge amount off time in a very localized environment. It's obvious that it would be very details if a series spends 20-30 volumes on just a single city. So, yes, Bookworm and Wild Game are extremely fleshed out after all this time (I even made fun of this in the Worldbuilding slide a while back).

The other way to think about it is by looking at the "believable" part of the question. I think here is Re:Zero one of the standout titles. I think talked about it before about how Re:Zero does this thing where there are in-universe stories and legends that are in-universe proverbs are build upon. It's very obvious with the dialogue of the Garfield character. He is constantly using proverbs that sound like utter nonsense to us readers but create the illusion that the world exists outside the viewers gaze.