As a quick recap, in Part 1:

We built a simple gRPC service for managing topics and messages in a chat service (like a very simple version of Zulip, Slack, or Teams).
gRPC provided a very easy way to represent the services and operations of this app.
We were able to serve (a very rudimentary implementation) from localhost on an arbitrary port (9000 by default) on a custom TCP protocol.
We were able to call the methods on these services both via a CLI utility (grpc_cli) as well as through generated clients (via tests).

The advantage of this approach is that any app/site/service can access this running server via a client (we could also generate JS or Swift or Java clients to make these calls in the respective environments).