I keep all my 50k+ photos on my Windows desktop and just loaded all the smart previews [but not the photos themselves] onto my Mac laptop. [transferred via flash drive--not the cloud]. From now on, how do I find the smart preview for new photos i add to my desktop catalog and then add them to the smart previews already on my mac?
For Lightroom-Classic: you cannot transfer Smart Previews anywhere by manual methods (flash drive, etc). -Well, you can in the OS, but they will no longer be associated with a Catalog or the Original Image files!
Transferring SPs (only) from a Mac to a PC is a totally useless exercise. There is no 'Sync' process of any kind.
Please explain exactly what you are trying to achieve.
Also list your LrC Version Number and info about your computers.