VCs can be created automatically if the user imports images directly (i.e. not by syncing) into one of the Lightroom cloud apps, and there is a LrClassic catalog which has been sync-enabled. Basically, once they have been uploaded to the cloud, the next time the synced LrC catalog is opened it will connect to the cloud and one of three things then happens regarding syncing those newly imported images to Classic:
1. If the images do not exist in the LrC catalog, the new imports are downloaded in original form and added to the LrC catalog, stored in the specified location, and added to the All Synced Photographs special collection (i.e. they will display the synced icon in Classic).
2. If the newly imported images already exist in the LrC catalog, then:
a) If in Classic they had already been synced as Smart Previews then the only thing that happens is that the cloud replaces the Smart Preview with the original, and Classic is made aware of that change, which can lead to the situation in some circumstances whereby Classic will sync/upload the full original again if that original is physically changed (e.g. by saving XMP to the file).
b) If in Classic the image had NOT been synced, then the new imports are made available to download to Classic. However, Classic's duplicates detection kicks in, recognises that what is being synced is in fact a duplicate of existing images and in that circumstance it simply creates a VC which is added to the catalog, and it's that VC that is the synced image that is added to All Synced Photographs. So in Classic you end up with an unsynced original and a synced Virtual Copy of that original.
Be aware that if you decide to remove the VC from Classic, that will delete the original that's in the cloud (the original in Classic should be unaffected). You can in Classic use the Set Copy as Master option which would make the synced VC the now-synced master, and the current unsysnced original would become the VC. Then you can remove the VC with no affect on the synced files in the cloud. The problem with this approach is that you can only do it one VC at a time.
When I was originally testing Classic catalog migration to the cloud, and then going against advice and re-enabling syncing of the migrated Classic catalog, I did experience some VCs being created as above, though in theory that shouldn't have happened. I don't know if that issue has subsequently been resolved (I doubt it, Adobe's advice has always been to not re-enable syncing of a migrated Classic catalog).