Keep photos on iCloud only

You can delete the photos from your devices, but first you'll want to go into your device settings>tap name>tap iCloud>tap photos>turn off iCloud Photo Library (iOS devices). On a mac, tap the Apple in top left corner>system preferences>iCloud>photos>select 'options'>turn off iCloud Photo Library. Once you have that turned off, you can safely delete all photos on your device without it deleting the photos/videos you have stored in iCloud. Many people do this to free up device storage.

Afterwards, you'll want to go back into your device settings>iCloud>tap photos and turn iCloud Photo Library back on (iOS Devices) or through System Preferences>iCloud>Photos>Options (mac). Select optimize device storage. If you don't turn iCloud Photo Library back on, new photos/videos you take will not upload to iCloud. By selecting 'optimize device storage', although you'll see your photos / videos from the Photo app, they will all not actually be stored on your device.

https://support.apple.com/kb/PH19449?viewlocale=en_US&locale=en_US

https://support.apple.com/en-us/HT205743

https://support.apple.com/en-us/HT204570

No, not if you use iCloud Photos. While Apple claims that iCloud Photos safely stores all of your photos, it is not an off devices ce storage facility. If you delete the photos from your phone, they are deleted everywhere. You should import the photos to a computer and then go an external drive and them delete them from iCloud is you don’t want them on the phone.

Archive or make copies of the information you store in iCloud - Apple Support

Import the photos with the cable.

Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support

If you use Google photos, you'll want to sign out of the app before deleting photos on your device or they will also delete in the Google Photo app.

This is incorrect. Google Photos is specifically designed to allow you to upload the photos to their servers and delete them from the device. It will even tell you when everything is uploaded so that you can then delete them.

See:

https://www.google.com/photos/about/

Never worry about running out of space on your phone again. Photos that are safely backed up can be removed from your device’s storage in just a tap.

Check out Google Photos for that feature.

If you use Google photos, you'll want to sign out of the app before deleting photos on your device or they will also delete in the Google Photo app.