Google runs three types of networks
Data Center Networks: designed from merchant silicon switches, with a logically centralized control plane; 
B4: A software-defined WAN that supports multiple traffic classes and uses centralized traffic engineering
B2: A Global WAN for user-facing traffic that employs decentralized traffic engineering

Three key principles that allowed Google to build scale out datacenter networks:

Clos topologies. The idea is leverage small commodity switches to build a non-blocking very large switch that can scale more or less infinitely.

Merchant silicon. Since Google wasn’t building a wide area Internet infrastructure they did not need massive routing tables or deep buffering. This mean commodity merchant silicon could do the job of building Clos based topologies in the datacenter.

Centralized control. Software agents knew what the network should look like, setup routing, and react to exceptions from the underlying plan. It’s much easier to manage a network when you know its shape compared to when you are constantly trying to discover what it should look like. If you want to change the network then you tell the centralized control software to change the network.