I know that there have been extensive trade networks, and at a minimum, awareness of other cultures, but I'd be curious how many "common people" of the age actually had any exposure beyond the goods brought back to their cities? Traders? Sure, they've met and interacted with other cultures. Military? Yup, Emperors and Generals and armies have found themselves on the fringes and beyond of their empires, but how much of that exposure made it back?
Well, not many- most people don't travel far from home, even now. I'm thinking of the merchants, traders, and sailors. How many people in Kansas have had exposure to Thailand culture, beyond the odd restaurant? If you are an adventurer with a pouch of gold, setting sail thataway over the horizon is not a hard task.
I find I have the most trouble when I try to do "all the worldbuilding" at the same time. I've recently been focusing on one area, or one country or region, fleshing that out, and basing my sandboxes there.
That's what I do; I build only what I need from what the players tell me and where I think they are going to go. Some aspects, like gods, magic, and geography, I have planned out in broad swathes, with some details. Magical techniques and the immaterial I find interesting, so I put effort there when I don't have to prepare something specific for an upcoming game. I've been thinking about it long enough that if someone pointed at a specific place on the world map I could likely give a one sentence description of what was there. But that's about it. There are two main places where campaigns have happened, so I have a lot of detail about a France sized area and an Ireland sized area.
Sometimes something comes to me and I write it down and think about it. Like, the city-state of Valon, run by the Hemalurgic Guild of magicians, has moneylenders at the gate. All silver coinage is exchanged for pewter coins of equivalent value. At least in Valon, anyway. Why? Don't know. How did the players find out? They found some pewter coins in a treasure trove. Every now and then they see the 106 pewter coins on their treasure sheet and talk about going there.
As it happens, I've since figured that out since one player has a PC from there and he asked what a Valoric insult would be. I told him that "silver sucker" or "coin licker" is synonymous with "coward". Regular people sometimes smuggle in silver coins or hackmetal and keep them in their cheek or under their tongue as a protection against a random curse from the ruling class.