When a monarch picks the kingdom, they also pick the biome that represents the Kingdom Seat. The monarch is then bound to the tribe that calls that biome home, irrespective of the demographics for their specifically selected duchy, county, or settlement. (Biome in this case means the overall biome -- as marked on the currently available maps -- and ignores transitional duchies or microbiomes at the borders between overarching biomes.)
The tribes are the ones I mentioned in relation to that part that snipe hunter explained in >this post<
Kairos’s community split so there is Kairos, which is called something else now I believe. And the Solaris confederation I believe it is called