After I imported photos into Lightroom Catalog "All Photos" I still have another folder that says imported photos. I wanted to delete the photos in that folder to bring it back to zero. I assumed that since I had already imported these photos it would be safe to delete them (those in the import folder) without affecting those same photos now in my Catalog "All Photos." However when I deleted these from the import photos my "All Photos" count dropped by the same number as I had in my import folder, also deleting them in my "All Photos". What am I doing wrong?
Leave "Previous Import" alone. Just let Lightroom Classic manage it. When you remove photos from Previous Import, this also removes the photos from All Photographs (and removes the photos from anywhere else you might see them -- collections, folders, publish services).
A Lightroom catalog does not contain the image files. It only contains a reference to where those images are located. You probably don't want all of your images in a single folder entitled, "All Photos". You probably would want to organize them differently, perhaps by year, or by subject, or some other method that suits your cataloging style. You can do that in the right-hand panel of the import dialog, designating a folder system as the images are imported into Lightroom. But if you choose to delete images then the images are deleted. If you choose to remove them from the catalog then they are no longer referenced by Lightroom but still remain in the folder, but all the adjustments that you may have completed using Lightroom will have been lost because those adjustments are what were stored in the catalog.