I cant really help because Im not great at explaining things, but for some fundamentals, you need to understand BGP and label switching (MPLS). The cleverness

I cant really help because Im not great at explaining things, but for some fundamentals, you need to understand BGP and label switching (MPLS). The cleverness here is that they effectively turned their routers in label switchers, with the label traffic is sent to determined by backend infra, not the router itself. They still have to run this over BGP of course, as thats how the internet works but it is an interesting approach for optimizing return traffic routes. Edit: espresso is the name of the stack they use to make the label switch decision, its not a protocol or industry standard