how do I delete photos/videos from iCloud but keep them on my Mac hard drive?

That is not how iCloud works. iCloud is a sync service designed to keep the libraries across all your devices the same. That means if you use iCloud on your mac, all photos in your synced mac library will also be in iCloud.

One workaround might be to have two libraries - one synced with iCloud. You can then transfer images from that library (via direct import on Monterey or later - or vie export/import in earlier MacOS) into the second library which is not synced with iCloud. Once transferred, verified and backed up, you could then delete the copies from the synced library.

You need to turn off iCloud photos on the mac, then you can delete anything you like in iCloud and the mac will be unaffected.

However, obviously you will then be unable to use iCloud to sync your mac.

Exactly what is it you are trying to achieve.