[quote=@Kafka Komedy]
Wait, so, it's never explained in the games as far as I'm aware: Is Termina part of Hyrule? Or is it just adjacent to Hyrule? Does it even exist? Or is it a product of Skull Kid or Link's imagination?

Basically, where are most of the Dekus concentrated? Cuz I have no idea where the Deku Monarchy would be located.
[/quote]

Termina is, as far as I can tell, a parallel dimension of sorts. That's the only way in-universe I can think of to explain why many NPC's and characters from OoT have dopplegangers in Majora's Mask. Plus, Majora's Mask falls under the Child Link Timeline, which is not at all the timeline we're following for this game.

As for the Dekus... I assume they come mostly from Kokiri Forest, perhaps they have some kind of hidden palace somewhere in the Lost Woods.