Teams must have an underlying Group but Groups can be independent of Teams.
Membership is consistent between a Team and its associated Group. Membership changes can be made in either interface. Note that there can be delays in replicating membership changes between Teams and Groups.
Yes. As a new Channel is created in a Team, a folder with the Channel name is created in the OneDrive for Groups view. Teams do not have the ability to see the root folder of OneDrive Group. You can move the data into the appropriate Channel folder if you want the files to be available in Teams. Note that the default Channel for a team is called “General”.