Perhaps I simply missed it somewhere, but when reading through Journeys Through the Radiant Citadel, I can't quite tell if the new civilizations it describes are entire separate realms of their own or located somewhere on one or multiple of D&Ds various material plane settings. Do Akharin Sangar, Atagua, Dayawlongon, Djaynai, Godsbreath, San Citlán, Sensa, Shankhabhumi, Siabsungkoh, Tayyib, Tletepec, Umizu, Yeonido, Yongjing, and Zinda all exist in Toril? Tayyib is supposed to be in a region called Suristhanam, but I can't find it on any maps of Toril or elsewhere for that matter. It does say at one point that "The Citadel doesn’t connect to all places in the multiverse—just the Material Plane locations linked to it by the Concord Jewels". Does that mean they exist across multiple material plane settings like Greyhawk, Eberron, Krynn, and etc. or are they exclusive to Toril? It also calls them realms a few times too. Maybe they're places that fall into the PHB's catch-all category of Other Planes or is it left to the DM to determine where they're physically located? I'd like to know in case I ever want to create a character from one of those places or if a PC wants to reach the radiant citadel.

I think the idea is that each one is its own world, not a part of an existing one. But it seems like it would probably be easy enough to drop them into, say, Toril. Just say it’s a land mass over this way, it’s not on the map. Of course, I haven’t gotten into the details, so it could be done wouldn’t quite work. 

Each civilization has a section in the adventure set there. For example, in Salted Legacy, which is set in Siabsungkoh, there is a section called Siabsungkoh Gazetteer

There is no fixed setting for each civilization / city. Instead each adventure has information on where to place it in a couple different settings.

Salted Legacy suggests Forgotten Realms and Eberron.