Server Design

I was wondering how massive servers like these are constructed. Does wynncraft use bungeecord to link all their worlds together?

 

I know development happens on a separate world and they have a custom tool to push map updates. There isn't a particular need to link worlds together, as far as I'm aware of. The system of increasing the number of available worlds depending on the player count was made around 4-5 years back, and as far as I'm aware has remained essentially untouched since then (besides changing the capacities occasionally).