What are the small details that makes the story feel like taking place in a living, breathing world?

All of my races have lungs.

Have some mistakes. People tripping, bumping into each other, accidentally saying the one thing when they mean another. Freudian slips, clumsy oafs. Have someone get mad over something simple and other people keep their cool under extreme stress, and vice versa for both. I'm a fantasy author myself, and let me tell you, the stigma of including relatable actions is really stupid. If you're running a DnD campaign, you don't have to worry about publishers telling you off for an off-hand mention that the main characters do indeed have bodily functions and do indeed use the restroom on occasion. So include it, because it will definitely make the world seem more natural.

There are a lot of small things that can improve a world. One example I like to ask myself is 'how does this race celebrate/party?'. What do they eat/drink, what festivities do they partake in, what do they do that gives them a feeling of excitement and happiness?

Cities and towns: Are streets paved? Is sewage piped or dropped out of windows? Do animals wander the streets or do they require leashes and clean up? How often does it rain? Are their city streets cleaned or does it pile up? These kinds of factors will affect the smells the characters deal with and that will form strong memories for them.

Diet and traditional foods. Every tribe in my world has one. For example, the Northern Ridge Tribe’s staple diet of roots, tubers, and flatbread flavored with spices and sauces reminiscent of the Earth Orient. They also have a custom of sitting before their food without eating for about fifteen minutes, to “awaken the stomach” and make the food taste better. And their food is always served by the children, whom they have a great respect for.

Due to their close relationship with the Southern tribe, they’ve also begun introducing fish into their meals as a delicacy of sorts.

There are several things to consider:

• what kind of foods/drinks are served? Who serves them?
• what times of day are they served?
• what dishes are they served on, and what utensils, if any, do they use to eat the food?
• what is done before and after eating the food? A prayer or blessing? A customary way of serving the food? Is it tasted by a respected figure first?
• is food shared between those eating, or do you keep your hands to yourself? Are meals eaten in groups, or are they solitary?
• is the meal eaten in silence, or is there conversation?
• what decorates the table? If there’s not a table, is the meal eaten standing, or in some other fashion? What is the seating like, if there is any?
• do wealthy and poor members eat different meals? What are the differences? What dishes have “crossed the divide” between different levels of wealth or status?
• are there any popular dishes borrowed or stolen from other cultures? What’s the history behind them?
• what’s one of the weirdest and exotic foods that makes you wonder how it was ever developed to begin with?

Et cetera.

And if you’re any good at cooking, you can actually try experimenting and designing the recipes yourself, in real life! (I am not, so I have to limit myself to my imagination.)

My "world" is in fact a way to make the stuff I build in Skylines feel more alive, so this is kind of the point. What do their houses look like inside, what are their favorite colors and fashions, what music do they listen to, etc...

One example I hinted at the history of my world, the humans of one nation strongly dislike snakes and reptiles, and will nail dead snakes to the front door of their homes for good luck and to ward off evil. This all traces back to an ancient Yuan Ti empire that enslaved the humans, so dislike of reptiles and snakes in particular is a part of the culture.

Everyone has an opinion even if I don't make it obvious and they're all over the place. From apathetic to disgusted across admiring to hateful. The whole world basically runs on opinions, from whether deities are real to whose politics are better across what food to eat (IIRC that's a plot point in the 1st novel). Cut away the story and action and it's just a bunch of people, AIs, and aliens arguing over every last damn thing. I don't think there are really two characters who ever see eye to eye if they meet. Even throwaway background characters have unique outlooks.

Every culture has an internal and external perception. Each is explored in broad strokes and day to day normality. These give the cultures character and internal conflict. Plays into the above.

World areas and epochs have themes but I try to constantly challenge paradigms and established tones. Nothing stays the same. If the same place appears twice it has always changed in the interim.

Though it often falls short, I try to describe little facettes in short but poignant detail. If I need a prop for a scene I give it a minor backstory. Even if it's silly or inconsequential. If I need a throwaway character, the same. Often the backstory loosely ties to "plot events".

Uh, that's all I can think of off hand.