How to access the gRPC API of Temporal Server?

If I have the Temporal Server running, what is the URL that hosts the gRPC API?

Are the proto file available through server reflection or hosted somewhere?

All gRPC communication between your applications and Temporal go through the Temporal frontend service.
You can find proto files in this repo.
What’s the use case where you need the proto files? Temporal SDKs and tctl do all that work for you already.