Hi Engineer Ted!
Good question! A few things to point out:
The answer to your question does depend on a few factors.
First:
Typically if you turn off iCloud Photos, you would be asked if you want to download a copy of them to your device, that way when you turn off iCloud, they are still on just that device.
Likewise, it depends on the storage and settings of your devices:
There are two main options for using iCloud Photos as for storage:
Download Originals, or Optimize Storage.
Keeping Download originals Enabled, as long as your device has enough local storage keeps all of the photos on that device.
Optimize Storage tries to save storage on your device by keeping the higher resolution photos in iCloud, not on your device, yet still lets you access them. In this scenario, if it was turned on, you wouldn’t be asked if you want to download these photos when turning off iCloud Photos.
If you DON’T have these photos stored locally or on any of your devices and disable iCloud Photos and don’t keep a copy.....the photos are still in iCloud, but would be deleted after 30 days unless you recover them before that time.
Here are a few articles that may explain it better than myself:
Set up and use iCloud Photos - Apple Support
Manage your photo and video storage - Apple Support
Archive or make copies of the information you store in iCloud - Apple Support
Get help with iCloud Photos - Apple Support
Hope something here helps!