Q: How to upload in the Photos iCloud only certain photo folders from Mac or iPad not all?

If you want to have only selected photos in your iCloud Photo Library you have to keep separate Photos Libraries for the photos you want in iCloud and the photos you don't want to upload.  See this Help Page:                 Use multiple libraries      

With iCloud Photos Library it is all or nothing - either all photos in system Photo Library will upload or none. By splitting the libraries, you can be more selective.