Thanks for your kind words :)
I will admit that in the rush to finish this in time for the jam deadline, I kind of skimmed over the living things in the City. I should probably go back and make a revised version, but in the meantime:
The City itself is entirely dead. Nothing remains of whatever life used to exist here; anything living has migrated in from another world at some point in the past. In the more sterile upper sunlit layers that generally means recently arrived travellers, but further down in the dark, fed by the nutrients of the vast subterranean ocean, that means ecosystems of algae, fungi, insects and the like; all originally hailing from other worlds, but now long since established here.
While someone stranded alone out in the dark might be able to survive off rats, mushrooms etc for a time, those ecosystems aren't enough to support whole societies. Hence, societies only show up in the deep City where there is some sort of source of more abundant energy - ecosystems of plants surviving in the light given off by the severed head of a sun god, a coven of witches using a mystic ritual to transmute food and water out of concrete, a one-way portal from a more living world, that sort of thing. (I foolishly included this as the column titled 'Energy Source' in the society table on page 12 without actually explaining what meant by that lmao.)
Beyond that, nomadic societies will tend to aquire their food and water, like all their other things, from either trading with or robbing settlements and newly-arrived travellers. I bet they'd pay a pretty penny for any long-lasting stuff like canned foods brought in by travellers, for instance. The nomadic vampire covens would tend to survive by preying on travellers, their lifestyle made possible by the relative frequency with which people can arrive through portals. Beyond that, most other beings who've taken residence in the City, like faeries and demons, generally wouldn't need to eat food.
I've rambled on for too long about this but in summary: resources like food can be found here, but they are scarce and will almost always be well-protected by the people who got there first.