It takes place in a sort of alternate version of Teyvat where the traveller either doesn't exist or just didn't wake up, but everyone is still in the same occupation they would be if he/she did. But, for me there is no singular static setting, since my main character goes from place to place depending on what his orders are for that particular section of the story. But his basis of operation is Snezhnaya in Teyvat (not by choice).
But, I try to use what knowledge I know about the unplayable regions in Genshin to my advantage, but embellishing it since we, for one thing, don't know what some of the maps even look like, so I take multiple creative liberties when the setting is Sumeru, Fontaine, Natlan or Snezhnaya with the itty bit of lore we do know, ie: Sumeru has a lot of deserts and rainforests, so for when the story was in Sumeru, I had it set in a rainforest. Natlan has volcanic activity, so I I say it's sweltering hot there (which could be canon), as well as Fontaine having fountains and being rich with water.
I'm limited with what I can do there, though.
For when the setting is Snezhnaya, I use the little bit of lore I have about the nation that was told by some npcs that are from there: Snezhnaya has month long blizzards, if you stand around you'll get frostbite and freeze to death, so I constantly say in the description paragraphs: "time is not a virtue in the permafrost north" as a callback to Childe saying in his voice lines, "where I'm from, if we stood around, we'd freeze to death, literally." So, I have to follow through on this as much as I can.
But, for the most part, almost all of the non-playable regions, I just have to do my own thing with as little available information to me as possible. But, the narrator breaks the fourth wall about it to say that in "an alternate version of Teyvat the area was not accessible to the general public" as a way to say it's not playable yet.
I deliberately chose for the "bases of operations" setting to be Snezhnaya. Honestly, a lot of the main fanfiction is in Snezhnaya for the sole reason it's not actually playable yet. I did this completely deliberately so I'm not limited to the playable maps. (Mondstadt, Liyue, Inazuma and Enkanomiya at the time of this post) So I can just do whatever I want. It'll be cool to see when all the regions are playable how either on or off the mark I was, or if I'm completely canon divergent. Time will tell.
So, I guess, in other words, I'm writing an alternate version of Teyvat where chimeras exist. But either
The Traveller isn't in the story, but events either proceed without him/her or just don't happen, probably. (The Cataclysm and Archon War still happened, but the Traveler wasn't there for the Archon war anyway). This is a Genshin fanfiction, after all.