Another worldbuilding application of the "two layer rule": To create a culture while avoiding The Planet Of Hats (the thing where a people only have one thing going for them, like "everyone wears a silly hat"): You only need two hats.
Try picking two random flat culture ideas and combine them, see how they interact. Let's say taking the Proud Warrior Race - people who are all about glory in battle and feats of strength, whose songs and ballads are about heroes in battle and whose education consists of combat and military tactics. Throw in another element: Living in diaspora. Suddenly you've got a whole more interesting dynamic going on - how did a people like this end up cast out of their old native land? How do they feel about it? How do they make a living now - as guards, mercenaries? How do their non-combatants live? Were they always warrior people, or did they become fighters out of necessity to fend for themselves in the lands of strangers? How do the peoples of these lands regard them?
Like I'm not shitting, it's literally that easy. You can avoid writing an one-dimensional culture just by adding another equally flat element, and the third dimension appears on its own just like that. And while one of the features can be location/climate, you can also combine two of those with each other.
Let's take a pretty standard Fantasy Race Biome: The forest people. Their job is the forest. They live there, hunt there, forage there, they have an obnoxious amount of sayings that somehow refer to trees, woods, or forests. Very high chance of being elves. And then a second common stock Fantasy Biome People: The Grim Cold North. Everything is bleak and grim up there. People are hardy and harsh, "frostbite because the climate hates you" and "stabbed because your neighbour hates you" are the most common causes of death. People are either completely humourless or have a horrifyingly dark, morbid sense of humour. They might find it funny that you genuinely can't tell which one.
Now combine them: Grim Cold Bleak Forest People. The summer lasts about 15 minutes and these people know every single type of berry, mushroom and herb that's edible in any fathomable way. You're not sure if they're joking about occasionally resorting to eating tree bark to survive the long dark winter. Not a warrior people, but very skilled in disappearing into the forest and picking off would-be invaders one by one. Once they fuck off into the woods you won't find them unless they want to be found.
You know, Finland.
