This is a great question let me explain it here,
So every time you create an office 365 group it has its own SharePoint and can create either a Teams or Yammer base on the needs lets say you created a team.
so all the members/owner will have access to the Teams, general channel, and any other channels created, same for the SharePoint the case for visitor/Guest users can be different where they can be granted access to general and standard channels and also on the SharePoint section and even restrict some or most of the files/libraries for Guests, Visitors, and even members.
Now the last one which is your case that is a private channel, every time a new private channel is created it does creates a separate SharePoint site which is also called child SP, and the members should be from the subset of the main team/office 365 group.
So in the private channel/SharePoint, you can have a member of the main team as the owner and not having one of the owners from the main team even as a member of this private channel / Sharepoint.