Maybe it's a "wormhole" but not necessarily tied to trash. It could be the old "things that we left around the house and never found them anymore" explanation, maybe linked by some inconsistency that makes small temporary dimension breaches on the space, bringing random objects regardless of use? But I feel like it maybe is something a little bigger that would explain not only the object appearing, but also the specific reincarnation of people into that world.
As for "it being tied to the MC seems unlikely". A Korean person (author of that book) has been there way before her, and the story relates more cases of people having memories of their past lives, so it's not a phenomenon exclusive to the protagonist that makes her uniquely special.
Edit: also the fact that there have been "souls" (or however the story will choose to call it) being reincarnated (transported) to that world is also why I think it's less likely for the phenomenon to be a wormhole tied to a specific location (like a dumpster). I think that whatever the phenomenon explication is, it probably wouldn't transport only objects (because reincarnation) and if it's restricted to a place, it would probably be a considerable amount of space in Korea rather than a specific location. Still, so far we can't know if all encountered so far being Korean is 1) Because the phenomenon is tied to Korean territory; 2) Pure coincidence and actually there's stuff from other places that haven't appeared yet; 3) Just narrative convenience and there will be only Korean stuff, but it will always be labeled as "stuff from your word" rather than explicitly pointing out everything is Korean (which is not as common in manhwa, as most stories I've read won't overlook that a worldly phenomenon wouldnt be restricted to a country, but is extremely common in manga and nothing stops it from being the case here).
