Well, a single server doesn't really need to do more than 10Gbps or 100k connections. Going above is a "simple" matter of managing horizontal scaling.

What I wonder about is how do you distribute the traffic on the higher level? I imagine there are separate clusters of envoys to serve different configurations/applications/locations? How many datacenters does dropbox have?

I was running a comparable setup in a large company, all based on HAProxy, there was a significant amount of complexity in routing requests to applications that might ultimately be in any of 30 datacenters.