Once you click Add X Pictures the sync appears to be as you indicate.
If you want only some folders at a time synced maybe add folders/albums in small batches. That way you have ultimate control of the synced file/album/folder structure.
What you ask sounds like a good candidate for a future feature request.
It means, every folder I create (and every picture I add) will be synced at this time?
Yes, it's a cloud client, so the central storage location is in the cloud. Selective sync may be added in future, but we have no idea on timescales.
If you don't want to sync your photos to the cloud, you may prefer Lightroom Classic instead.
Thanks for the answer.
But without making selections, this would go beyond our bandwith and usabillity.
We tried LR Classic, not really our cup of tea.