If I’m not mistaken, race usually refers to the color of one’s skin—including specific features of their body. Ethnicity refers to the culture in which that person belongs in. Nationality refers the country in which that person lives in. Though, some may use the term “race” to identify all of these which isn’t always a bad thing because in many ways, it can be true. Unless that person immigrated to another country and or doesn’t always identify with their own culture and heritage, especially if they are interracial.
But this can be the same thing for fictional beings who are not humanoid and are not humans (and vice versa). You have to identify the beings in your story, what sets them all apart (within their physical traits and cultures) and how they come together in your story to form a community.
So, for example, in my sci-fi fantasy where there are alien species (humanoid and other) living among humans, I had to create a backstory for where they come from, their customs, their cultures, traditions, and so on. One of them is called Koelegian (Koh-leg-ee-n) where they are from a planet far away called Koeleg. They have gods and goddesses, who are part of their religions, and they have music and food and other things that are very different to other species. Their technology and the way they live is also different as well.
But they do not live together, technically. Some of them know of each other, may have met, but live in different countries on Earth. However, if they did, it wouldn’t be as different as we have now.
Going back to the human analogy, some places are very cultural. Places like Mexico, South America, or in the Middle East all have very different looking towns, neighborhoods, cities. Some may be “normal” looking, like nothing different to many other places. But I’ve seen pictures of towns and villages in places like the Middle East where there’s poverty or they use different materials to build houses and buildings. Same for places like England or in the UK where many buildings are all still made of brick and stone, looking like it once was in the Victorian (or earlier) times. But go to somewhere like America, and you get a bit of a culture shock with housing and buildings and where things are located. How things work. But places like America are more culturally diverse because people from other countries immigrated there, and built things and are part of specific communities that show more of that culture than anything.
Like, for instance, I live in a cowboy town in Kansas. Middle of nowhere really, but it’s built on cowboys and outlaws from the 1800s. These cowboys though probably weren’t all white folks, unless many immigrated here, because the majority of the town’s population are Mexicans, though some are Cubans, Puerto Ricans, and South American. But most people are either Spanish speaking or bilingual (Spanish and English speaking), most local businesses are Mexican-owned and Mexican-based, etc. And this is similar to some places in California since California used to be part of Mexico. But if you somewhere else where the population is split between multiple people of other cultures and backgrounds, you get a bit of everything. In Columbus, Ohio, for example, there’s a lot of different kinds of people and businesses and activities to do and architecture. German Village looks different to Easton, and Easton is different to Westerville, etc. With the population being just under a million, you do get a lot of cultures blending together.
You can also think about what it would be like if two people from other cultures, other ethnicities, other backgrounds are like when they come together and form a family.
My sister, who is very white, married a Mexican. My sister is not religious. She doesn’t celebrate many holidays besides birthdays, Halloween, and Christmas. She loves food and loves to experiment with different things, and while she loves Mexican food, it isn’t the only thing she eats. And she doesn’t have much of a heritage or main traditions (mostly because my parents never really did much with us—partly because we hardly know much of their history). But his family? They have lots of traditions, lots of holidays, they’re religious (Catholic), and while they eat a lot of various foods, I know that his mom mostly sticks with their own cultural food (at least, whenever we eat with them lol). And when they have children, they will have to figure out if they want to do these things or if they want to raise them differently, or like most people, to give a blend of it and or introduce it but not force it.
In a nutshell, you’re pretty much trying to figure out these parts of these people and how these traits help flesh these characters out and make them come alive.
Also note that not everyone from every background is going to continue traditions or do the same things as others in their cultures do. Just because you grew up in a certain culture doesn’t mean you are forced to then continue being a part of that culture. Many do because it is their heritage, it’s in their blood. But there’s this thing called free will where they are able to not like certain things and be different or move somewhere and not do what their family does every year or every month. You are also not obligated to showcase these pieces of your background to your children or friends if you do not wish to. And some people, like myself, don’t have specific cultures, but I also live a different life than someone who may be Polynesian or Asian or Nigerian or anyone else. But that’s what makes the world interesting: not every place is the same.