The Nature of Sentients

Sentients, also known as races in standard Dungeons & Dragons terminology, are bloodlines, heritages, species, or ancestries a character can belong to. Since the central narrative location of Hillit is in the United Halla Alliance, sentients follow the organization's classifications and terminology.
Many sentient groups have an array of ethnicities within them and across them -- a subgroup of people who identify with each other through common language, history, society, culture, or religious practice. Some ethnicities are fairly distinct to a sentient category with a strong ancestral line and can have their own mechanical distinctions; this would be a subrace or variant race. Other ethnicities are more culturally defined and can encompass multiple kinds of subraces or sentient races.
For ease of navigation, the sentient races are categorized into five groups:

• UHA Common Sentients are the most significant populations in the United Halla Alliance. These races are dragonborn, dwarf, gnome, halfling, human, kobold, and tiefling.
• UHA Uncommon Sentients can be seen in metropolitan areas and districts of multicultural cities, but they are not very frequent. These races are eladrin, elf, half-elf, half-orc, lizardfolk, mnotaur, and shifter.
• UHA Rare Sentients are not frequently seen and are usually found within their own more isolated communities. These races are bugbear, firbolg, genasi, goblin, hobgoblin, and kenku.
• UHA Very Rare Sentients are, as the name suggests, very rare. It is quite possible that a citizen of the United Halla Alliance may never run across a sentient race in this category. These races include aasimar, changeling, drow, orc, shadar-kai, yuan-ti, and potentially other rare classifications.
• Non-UHA Sentients are sentient classifications that, in the latest population census, do not appear amongst the citizens of the United Halla Alliance. They may be found in other locations on Hillit, or one of the planes of existence.

There are challenges with the terminology here. Many of the "races" of dungeons and dragons are realistically the same species -- two members of different races are naturally able to produce a fertile offspring. Not all race combinations are capable of this.
Frequently, in Dungeons and Dragons, many subraces (and sometimes whole races) are represented having a single culture. This is not always true in Hillit -- biological subraces (like drow vs eladrin) should have in-game mechanics regarding their biology (senses, speed, etc.). Other subraces can represent a member of that race with a specific cultural background (hill vs mountain dwarf).
I favor having races featuring biological traits and subraces featuring cultural traits. However, I know it can be problematic in D&D to have such strict racial definitions and cultural classifications. Many animalistic and monstrous races can read as the horror fantasy image of a Victorian English author's perspective of another Earth culture -- hobgoblins are roving Huns and Romans who raid and raze indiscriminantly, lizardfolk are African and South American tribes with non-relatable psychology and cannibalism, sahuagin are Polynesian tropes codified by Lovecraft, yuan-ti are Mesoamerican horrors for Cortez, and so on. I want to divorce this stereotyping to the best of my ability as a worldbuilder, but I know that I only have so much perspective on cultures I am not a part of and that can be difficult to include. I don't want everyone to be just my culture and I don't want my attempt to include other cultural inspirations to result in cruel stereotyping. This is a serious topic for me and I will constantly try to improve my writing regarding it.
Ideally, we would have a race and a subrace to represent a genetic lineage, then a cultural background, then a professional background to help build up a character's place in the world.

Prejudice amongst sentients

It is a natural part of society to form groups with similar beings -- this drive for community is what shapes civilization from the wilds of history. Sadly, "similar" is a vague term and can be quite limited in some circles.
Most of the United Halla Alliance tolerates members of other sentient races, but there are certain prejudices that are spread, especially for uncommon and rarer sentients. Without frequent exposure of other ways of life, it is easy for a member of the common sentient races to make wide assumptions and not have them challenged meaningfully.
In each of the sentient pages, the beginning section (i.e. the vignette) contains broad understandings of that sentient race by outsiders. It may be inaccurate or contrary to how the sentients view themselves, but it is still a fairly common presumption by sentients outside the category.
When you play in Hillit, you may have members of society assume qualities about you or treat you differently depending on how you appear. This is wrong of them to do and these NPCs might act from a place of hatred or ignorance. Your character may also harbor assumptions about those around them, for many kinds of reasons (e.g. prideful privilege, marginalization, victim hood, upbringing). Ultimately, it is up to you to decide how your character understands the "other" in the world.

Mixed ancestry and children

Players may want to take on a character that comes from different lineages. This is a frequent desire -- a child of "two worlds" makes for an interesting narrative. It is possible, but only naturally with certain sentient races.
As seen in the Soul Categorization article, a sentient is born with a certain type of soul. Biologically, unless otherwise specified, a child can be born from most unions within a soul category. For instance, an orc and a human could potentially produce an offspring (in this case, a half-orc). If a sentient race for a mixed ancestry isn't present, then their offspring might be counted as a type of ethnicity (and mechanically be a subrace or a variant race of one of the parents) or be so infrequent as to not have a standard classification.
Infrequence mostly comes from simple biology: two people of otherwise compatible souls might not have a high fertility rate between each other. This is often due to physical limitations, such as the egg-laying of a lizardfolk being incompatible with the live-birth of a hobgoblin.
Two members of two different sentient classifications that are able to naturally produce a fertile child are considered members of the same species, but the term 'species' is not frequently used in Hillit to describe large groups of sentients.

Magical solutions

There are magical solutions to produce a child of an otherwise impossible ancestry. The magic required is severe, difficult, and dangerous, for it requires the shaping of a body in the womb, the stealing of a soul to implant, and the binding of the soul to the body. This mixture of transmutation and necromancy is forbidden in the United Halla Alliance.
There are also more gentle magical solutions, such as potions of fertility, which increase the chance of having at least one child. Many times, these potions result in twins or triplets. These potions are controversial since they seem to weaken the mother's body during the pregnancy and the mother birth death rate is higher than normal.