Setting Notes

While this world has many similarities to ours, there are many fundamental differences. These differences will occasionally be highlighted in-article, but are universally present. For each of these major changes, I will get really into the nitty gritty and justifications, and then do a little "tl;dr" to actually read.

Trade, Technology, and Connectivity

The Change: Our world was never as isolated as we imagine it to be- trade lines spanning Eurasia predate cities, and technology develops in cross-continental blocks rather than isolated regions. But this world surpasses even our own in its interconnectivity. 
There are several reasons for this. Firstly, Halika lacks the massive oceanic divides of our world. There are no isolated Americas here to keep maize out of Asia. Secondly, The Lunar Pantheon regularly communicating with mortals around the world means that developments can jump around the world in a matter of hours or days. Technologies can also be artificially transplanted by the lunar pantheon as well. Thirdly, magic allows for rapid communication and transit for elite spellcasters and those wealthy enough to purchase their services. And lastly, different species living in different environments- notably Prisms and Half Prisms easily living in inhospitable mountain ranges- mean that certain less-hospitable regions are more populated and easier to trade through.
This is self-reinforcing. Technology sharing and transplant means that transportation technologies accelerate worldwide, meaning that the already-smaller geographic barriers become even easier to overcome. Lunar guidance also means it is easier to know if there is a route, passage, or wealthy destination. Magical worldwide leaps of elite powerholders mean that elites can introduce fashions and tastes that require longer voyages, rewarding technological progress and encouraging faster adaptation. The list goes on. 
The Limits: This isn't to say that all regions know everything about each other and travel is easy. The Lunar Pantheon can often twist international news, news is filtered down through the elites who often don't bother to share it with anyone below them, and the greater presence of magic means that people are more willing to believe absurd myths about foreign peoples and events. And access to things like better ships or compasses or printing presses is not universal. For example, in places where the infamous Selkie merchant-empire known as The Khilaia rules over the seas, local merchants often lack the incentives and resources to start their own international trading convoys.
Similarly, not all things are shared readily by the Lunar Pantheon and not all things are considered important at the time. Things like Alchemy, Magical Arts, or how to make Empty constructs are so difficult to explain in a few minutes (or even hundreds of few-minute conversations over the course of decades) and so dangerous if handed out that the Lunar Pantheon has agreed to avoid them entirely. On the opposite end of the spectrum, things like foreign customs or religions are often seen as irrelevant and are more useful locally as exotic props and literary devices. 
As for technology: Technological transplants are often messy, as technologies are more than inventions- they require institutions to actually mass produce or use those inventions, and who has power over those institutions and who doesn't is always a battle. A good example is the gunpowder culture of Suneka. The realms of Suneka share a common religion that emphasizes public education and uniformity, making mass production of finnicky gunpowder weapons easier. So while the idea of gunpowder projectiles has traveled around the world, only master craftsmen can make reliable and effective ones in most places. The Suneka, meanwhile, has the educational and economic institutions to really mass produce high-quality firearms. And even the Lunar Pantheon doesn't really recognize what is so special about their firearms being better, so the details of certain technologies (ie flintlocks) have yet to be transplanted on a large scale. 
So, What? Basically trade is bigger, more places have access to more animals and crops, technologies can jump faster, individuals can flutter between continents, and places have better maps. But it can be really uneven. 

Population and Food

The Change: The architects made many sentient species, and when they did so they suddenly multiplied the total population of the planet by at least four. These species eat different things and draw on entirely different resources. Some are omnivorous, some are carnivores, some eat minerals, some eat detritus. Some are susceptible to different diseases than others, while some have no natural diseases at all. And so these populations were able to grow much higher than they would in our world, and much denser. 
Plagues are also less devastating in this world. They rarely effect everyone, and its much easier to find healers who aren't in danger of being infected. Outbreaks are much less dangerous when they don't come with system collapse as well. Famines rarely effect everyone as well, as food sources are varied. That variety also makes recovery from war or natural disasters more expensive but the resulting population rebound is faster.
On a related note, the variety of species also makes no land truly uninhabitable. Someone fill find something to eat anywhere here, and some species are just better suited for tough climates: no longer are freezing cold taigas or huge barren mountain ranges a problem for settlement (for the right sentients)
The Limits: Food variety requires infrastructure variety, and sometimes a society prioritizes one over another. Sometimes this choice is one or the other: arable land can be used for meat, vegetables, fruit, and insect farming but which gets the majority of the land and water? You can use land for multiple, but certain layouts and techniques produce different ratios of food usable by different species. Sometimes, this choice is a resource question: how much effort should be expended by non-Prism communities to gather labor-intensive Prism food? The answer to these questions of who should have their food production prioritized can limit certain species growth rates while bolstering others, reducing the total population boom given by everyone multiplying at once. 
Further complicating this is the unequal growth rates of species: some populations just breed faster than others, meaning that not all increases in food production translate to equal increases in population. So, basically, its a mess. Add in hybrids and the calculations are a nightmare. 
So yes, in an ideal world the population would be insanely large and dense. But there are a lot of inefficiencies that can be locked into systems of power that damper that. 
Lastly, there is the issue of contraception. It is way more efficient and common in this world than ours, and much harder to regulate. It is easily grown in a hardy plant that is found around the world- large families are a lot more voluntary here. Rapid population growth in Halika requires purpose - economic, ideological- and things that disincentivize more children (food prices, lack of opportunity) hit the growth rates harder because children are easier to not have.
So, What: Populations of regions and cities are larger in Halika because there is more food and less disease. Easy access to birth control means there needs to be a reason for enormous families, and systematic preference for one species over others easily sways population rates (typically reducing them). 
As species are unequal in growth rates, geographic origination, and food availability, species populations are unequal. This breaks down different region by region, but globally it is: Dryads as the most common, followed by Humans, followed by Prisms and hybrids. 

Taboos

The Change: The variety in sentient species in Halika has led to a fragmentation of taboos humans consider common. While humans are not uniform in our taboos (in fact, every taboo listed below has examples of cultures that don't follow them), certain ones have been associated with large agrarian power structures and societies. This is less true in Halika.
Cannibalism, Corpses, and Decay: So, not all sentient species are omnivores, and not all of them are plagued by disease. Not all of them have senses of smell or even sight. This means that the natural revulsions consistently found in humans are not universal here. The big one is cannibalism and corpses. For Dryads and Prisms, a corpse is truly closer to food than a biohazard. Prisms in particular have almost no danger cannibalizing each other, as non-magical Prisms diseases are nonexistent. Dryads and prisms also are less repulsed by rot and decay. For dryads, rotting food is edible. Prism bodies not only don't rot, but a prism wouldn't even be struck by the scent or colors of rot. So many ancient Prism and Dryad traditions include cannibalizing the bodies of dead community members- not only are corpses not repulsive to them, but their food is closer to their dead. For dryads, the process of decomposition and the role of life forms in decomposing matter is unavoidable- the dead are not preserved in tombs or disintegrated into the world, but must be eaten by something to pass on. And a Prim's body is similar to nothing besides Prism food- a comparison that becomes incredibly clear when said Prism dies. So Prism and Dryad communities will bring those cannibalistic traditions with them and normalize them. 
Gender and Sexuality: Gender and sexuality taboos require classification, and classification requires the idea of uniformity. Gender and sexuality are not uniform among humans, and that variety increases when you add fantasy species. Dryads lack all secondary sex characteristics and can have different or both sexes contained within their flowers. As such, dryad-only societies often lack the idea of gender and fashion their gender-ambiguity into a third gender when incorporated into human-majority societies. Dryads normalize gender non-conformity by their very presence. Prisms are the same way. Solars lack sex entirely and also reproduce in threes. Kobolds have only one sex. These undermine gender-based roles and taboos. The ability of Kobolds and Hybrids to interbreed with humans carries all of this into human communities as well. The damage done to gender taboos carries over to sexuality taboos- where one is undermined, the other follows. Add in easy-access birth control that reduces the danger of inconvenient pregnancy and normalizes non-reproductive sex? Suddenly, gender and sexuality taboos become much harder to justify and maintain. 
Nudity/Clothing: Dryads reproduce by flowering. They flower on their head. What does modesty mean for a dryad? Pants, out of principle? A shawl or veil? Meanwhile, if a Solar wears clothes, they lose out on food. Clothing and anatomical meanings are less constant here. 
The Limits Most of the taboos listed above were already artificial, and artificial structures can be revised and imposed even when they don't have logical consistency. Some places will have some of these things anyways. While taboos must be negotiated place-by-place, they haven't dissapeared. For example, the religion and culture of Suneka has refashioned gender to be entirely role-based and divorced from physical characteristics entirely- gender taboos remain, but mean something totally different now. And just as some species can normalize some previously taboo topics, these species can also make those things taboo for those they meet. Often, its a negotiation where both realities can exist at the same time: in many societies cannibalism is an extreme taboo in most scenarios, but is made normal in certain ceremonies or rituals. 
So, What Cannibalism is a much more accepted taboo in many places. Gender, sexuality, and body norms vary a lot from place to place, so don't make assumptions (even more so than in real life, which was already super varied). Modern ideas of misogyny, homophobia, and transphobia are extremely rare.