The thing about Google Backup is that it's used primarily to restore to a brand new phone (so you'd have to factory reset the phone and then go through the setup wizard to make use of the Restore function).
The way Google Drive works, the files should be synced on a constant basis from your phone or computer to Google Drive in the cloud (assuming, of course, that these files are saved to Google Drive, or in the case of a computer, the folder that the files are in was designated to be synced to Google Drive). So I don't think those files would necessarily be part of the Google Backup process, since they should already be backed up/synced anyway.