Yes, the definitions of the workspaces are stored in the "Preferences" file. What tabs are in each workspace are stored separately in the files in the "Sessions" folder.
The workspace definitions are synced via syncing settings and open tabs are synced to the server via the separate open tabs sync setting where they're just made available to manually open at the URL opera://activity without any context of workspaces and tab groups.
Things might improve someday hopefully where opera://activity is workspace-aware.
On a side note, be careful when syncing settings between 2 Opera's that have different workspaces. That might confuse Sync and set all Operas to use the same workspaces while in the process wiping out others.
