That level of scale always boils down to partitioning work. The fact that these partition are distributed across regions is done for risk mitigation (e.g. what

That level of scale always boils down to partitioning work. The fact that these partition are distributed across regions is done for risk mitigation (e.g. what happens when AWS's eu-central-1 catches in fire). Architecturally, nothing really changes if you stuffed them all in the same region.