I still need to gather up all the evidence here, so for now you get a mostly blank page where I will just say that so much of this story is about fertilizer. In my head, it kinda works like this:
flowchart TD
A[Land in Teyvat is infertile] --> B[People can't grow food] --> C[People turn to Alchemy to create food] --> D[Eating food created with alchemy causes corruption] --> E[When people die, their corrupted bodies further pollute the land] --> A
With what we know from Records of Hanan Pacha, there’s the added detail that eating food grown from Teyvat’s soil tethers you to the Night Kingdom, so you can’t leave.