Building cultures that feel lived-in requires moving beyond the 'fantasy Europe' buffet. Many isekai just paste in elves and dwarves, but the worlds I remember treat culture as an operating system. Take 'Ascendance of a Bookworm'—the protagonist's entire struggle is about navigating a rigid medieval-esque class structure and guild system, where literacy is power and paper is a luxury item. The magic isn't just spells; it's in the social rituals, the economic dependencies, and the unspoken rules she has to decode to survive.
It's not just about describing festivals or food, though those help. It's about showing how those things affect daily logic. In a well-built world, the culture dictates the conflicts. If there's a strict mana hierarchy, how does that shape law, romance, or commerce? Authors who succeed think about infrastructure: how do messages travel, how is justice administered, what do people genuinely believe about the gods? The culture should present obstacles and opportunities that feel organic, not just convenient for the plot.
Honestly, I get bored when the 'culture' is just a thin justification for the hero to show off modern ideas. The immersion breaks when everyone instantly accepts his democracy lecture. Real immersion comes when the culture pushes back, when the protagonist has to adapt, compromise, and sometimes fail because the world's logic is different and deeply rooted.