go-zero is a wrapper built on top of gRPC. gRPC uses ProtoBuf as its serialization protocol. First read the ProtoBuf documentation, then the gRPC documentation, and finally the go-zero documentation.
There is a bug in goctl quickstart on Windows; do not use it. It will not generate go.mod under the greet folder, but will instead generate go.mod separately in the api and rpc folders.