Only a single CloudDB is required. Read about how the CloudDB works. Make a simple Project with a chat (and without your Canvas game) and learn how the real time database works. Read the links provided and do the tutorials.
This chat app Social Distancing ... a CloudDB / Location Marker Tutorial shows one way to select the users who can post information (in your words, the 'controlling player')
If you learn to only copy code and do not learn how to use the tools, you are going to be totally dependent on others. So maybe you can learn to use the tools?
Show the community what you coded if you run into trouble and someone might provide specific advice.