Introducing the Community Creation API for Viva Engage on Microsoft Graph Beta

With some minor struggles I did make a Viva Engage community.

I found that I had to set the owners parameter without the ('MyIdAdObjectID') single quote and brackets.
example had:

"email address removed for privacy reasons": ["https://graph.microsoft.com/beta/users/('MyIdAdObjectID')"]
gave error "Empty segment encountered in request URL. Please make sure that a valid request URL is specified."
This worked:

"email address removed for privacy reasons": ["https://graph.microsoft.com/v1.0/users/MyIdAdObjectID"]

Now I want to retrieve posts or conversations to that new community. I've searched for documentation or examples or Copilot advice, but can't find anything. 
To be clear, I'd like to programmatically get all posts to a Viva Engage Community.

The first step would be to get a list of all my Communities. I can't even find how to get a Community ID.

GET /employeeExperience/communities/{communityId}