Fundamentally, anything that's a clear historical-cultural analogue is something that got changed by or pasted on to the Realms by TSR. Greenwood mostly developed the Sword Coast, Moonsea, and Cormyr/Dalelands/Sembia.
Lands beyond that were either undetailed or vaguely sketched exotic lands for NPCs to be from. So Thay, for example, was a mysterious empire where various meddling Red Wizards encountered by his players came from, with Greenwood's typewritten notes (some of which were published in Ed Greenwood Presents Elminster's Forgotten Realms) suggesting that it would be a suitable region for Oriental Adventures material. Calimshan was similarly explicitly called out in those notes as being "lightly detailed"; it was mostly a place for exotic things found in the detailed parts of the Realms to have come from.
But TSR from 1985-1995 was really, really big on direct real-world adaptations, seen in how they handled the Known World (Mystara), in the green-covered HR series of supplements, and in what they did with the Realms. So TSR made the Moonshaes into Fantasy Celtland, and Mulhorand into Fantasy Egypt, and Unther into Fantasy Babylon, and Chessenta into Fantasy Greece, and appended the Hordelands and Kara-Tur to the east, and added Zakhara to the south and Maztica to the west, et cetera.
