AFAIK they have a bunch of production infra on protobuff/gRPC - not so sure about flatbufferrs which came out of the game dev side - that's the difference maker to me - which project is actually rooted in.
If you worked on Go projects that import Google protobuf / grpc / Kubernetes client libraries you are often reminded of that fact.
Stubby, not gRPC. Stubby is used for almost everything internally. gRPC is a similar-ish looking thing that is open sourced, but not used nearly as much as stubby internally.
Stubby predates gRPC by like 15 years or something.
> not so sure about flatbufferrs which came out of the game dev side
I wouldn't know. I'll be honest, I always forget that Google made flatbuffers. I guess if you're doing a lot of IPC?