Lynnebk10 wrote: … what I’ve read online is that if you turn off iCloud syncing that they delete?
Sometimes people have pictures that aren't actually in the Photos Library and aren't controlled by Photos-- perhaps because they were transferred manually. Those can be problematic.
The other thing is, if you have "Optimize Storage" turned on, then Photos relies on the originals in iCloud. Disconnecting from iCloud would leave the Mac with just smaller preview files and no access to the originals. In that case you would be given the option of downloading all the originals, or deleting the previews. If you see this notice, be sure to read it carefully, and be patient in asking questions to be sure. I do not use "Optimize Storage" on my Mac-- but I do use "Optimize" on my iPhone, since it has way less storage.