Due to lack of land, there is no agriculture in Orth. This means that the town's food can only come from two places:
If Orth was trading relics for food, we would be seeing a constant line of ships coming and going. There would also be infrastructure in place to transport goods up and down the outside of the volcano.
Hunting/Foraging in the Abyss
In Episode 5, Riko mentions that any meat they had at the orphanage came from here, so this is the likely option. There are thousands (possibly tens of thousands) of people living on the surface. Shouldn't there be hundreds of people in the first layer every day hunting animals, catching fish and gathering fruit? How come we never see anyone other than the odd cave raider?
As much as food plays a role with our main characters, we curiously never see anyone eating on the surface. I doubt that the need to eat is tied to the Curse of the Abyss.
In a story so world-building focused, I find this to be a strange oversight (though not one that draws from my enjoyment of the show).