So do Unliving have a working digestive system? What kind of changes allow them to eat vomit and poo? Mold? Where do they get the body parts for human-based foods?

Some strains don't have eyes. Can they see?

Do they have a circulatory system? How do they deal with rotting body parts otherwise?

Some other food to chew on: some strains are more like talking animals. We don't know all the strains in-world, and neither do the Unliving (seeing as they were convinced by SAI and vial infections). Death Slug, Doom Bloom, and Barnankles are examples of plant/animal like infections that resemble zapper skins. Who decides what counts in the Underground? Would anyone fake it by getting zapped?

Do you mean to talk about what kind of lore everybody made up by their self about the strains and each zombie?

Dubbel-post by glitch.

The idea that zombies eat things, like legitimately consume them, never really made sense to me. If the virus is transferred via saliva then zombies would want to bite their victims a bunch, but not outright eat/kill them.

I like to think that since food shops have been established here in the underground for subeta zombies mean that things are different. Viruses taking over or reanimating the body makes me think of zombified insects that only act on the virus' accord to be eaten, which is not the case here either. The virus does spread itself to others but maintains itself within the original host as well. The virus is still focused on propagation by making zombies want to bite other people, but why would it need them to eat? Since zombies in subeta are sentient, I'd like to think it's their choice, not the choice of their virus, to eat. Maybe eating human parts tricks or distracts the virus into thinking its a living victim with living cells that can be targeted, and dulls down zombies' urge to bite actual people, so that they can remain at peace with those above ground. Maybe they just want to eat but don't need to because after being human for so long there are just habits that they don't want to give up.

But how to they process the food? Does it fall out? Get dissolved, absorbed, and utilized by the virus somehow? We really have no idea on zombie biology, how and if they even have metabolisms, and ways to become a zombie seem to be pretty diverse, so it could work differently for each virus. I'm going to say that most probably don't need to eat as a necessary function but can if they want to experience the novelty of it. I'd assume taste buds are still a very prominent feature somehow? Unless they just choose what they want to eat based on how it looks. Maybe in Subeta's reality their digestive system still works as you said, and it could just be that simple. I personally believe nothing actually works down there anymore, but the eating factor really is a bit of a conundrum.

To get parts, maybe they either make deals with Quentin, or even directly through SAI to receive a steady supply of body parts to keep them content and amicable. I'd imagine after 2009, SAI would want to maintain good relations with the underground at almost any cost. Either publicly through laws that make the exchange legal, or through backdoor deals so as not to provoke citizen outrage.

Eye-less strains are probably either blind, have other heightened senses, or a developed non-human sense, like being able to perceive infra red or ultra violet or something? Echolocation? Like they can still 'see' but not in the conventional human way.

Rachelle does the majority of professional fixes for rotting body parts. Maybe even Weeper has a few remedies for holding parts together?

Though the Underground has their own government, we haven't seen very much of it since 2012. Who knows what laws they have in place to decide what constitutes as a zombie? I'm sure there are definitely some strains that receive prejudice, such as this description indicates:

However I think Gunner's VIP list is very public in the underground. Maybe Skitters and Ecks have a set of guidelines that can be referred to of what makes a zombie versus other undead. Shadowglen itself probably has a very informative codex lying around in that regard? Or maybe the research just hasn't been done yet.

Zombie strains have kind of gotten more grey as the shift from diseases resembling real afflictions to made-up ones have taken place. Stuff like death slug just looks like a giant underwater sea slug, how would the average person know it was undead or formerly human? Not everyone would be well read on that sort of topic, and perhaps assumptions are made by ignorant passersby? Skitters' zombie den does sell the plushies, so I'm sure underground residents would recognize them as zombies, but maybe not so much the general living populace. As for zapper skins one might be able to get away with looking monstrous or unusual for a little while, but I think eventually certain physical behaviors would indicate their alive status and expose them. Not that it's against the law to be visiting the underground as someone who is living, but the attempted impersonation of a zombie would definitely be frowned upon, and probably very offensive. If they wanted to live in the underground and remained reclusive, maybe no one would really interact with them enough to care, but so far I don't think any living person has taken up residence down there? Though there are plenty of animals, like the bugs and fish.

While I don't have anything to really add to this i'd like to mention: I don't think all zombies spread via biting. Like clusterflux is spread by maggots- something their own biology probably produces, same goes with Hivemind.

Muerte Fuerte by punching

Bakers kiss with a kiss etc, etc