I have created 2 separate projects in Firebase one for the TEST (development) and one for the actual PROD(unction) environment.
I have create a hosting project and am also using Firebase functions that I have successfully deployed and tested on the Firebase TEST project (using the command line as described in the docs).
What is the best / easiest way to now publish to the Firebase PROD project?
Get familiar with the way the Firebase CLI lets you attach a workspace to multiple projects. You can use firebase use --add on the command line to add a project alias, then firebase use [project] to switch between projects for deployment.