If I clean my iPhone photos, will it erase my iCloud Photos?

If you sync with iCloud, all of your photos sync across any device on which you have iCloud Photos enabled. If you delete a photo, it deletes everywhere.

You can elect to optimize photos (settings > photos) to reduce the size. of your library on device and keep originals in iCloud.

Help-community-for-Apple wrote: …I fear that during SYNC anything I delete on my phone will get deleted from iCloud.

This is exactly what "Synchronize" means. What you do in Photos on your iPhone or Mac or iPad or iCloud.com is also done on all the others.

You have two options: One is to turn on "Optimize iPhone Storage" on your phone. This causes the phone to keep smaller versions of the pictures on the phone, but it will grab the larger versions from iCloud when they're needed. It can save as much as 90% of the space that Photos takes up, depending on the "need." It tries to keep more original files when there's plenty of room.

The other option is to save pictures to a computer or other hard drive.

Import and export photos and videos on iPhone - Apple Support

Saving to an external drive is way easier on the newer iPhones that have USB-C ports. For phones with lightning connectors, my luck has been more iffy. You may need to a get a flash drive made just for this.

Transferring pictures directly to a computer using the USB connection won't work. The USB "sync" is incompatible with iCloud Photos synchronization.

(I swear, muguy's post was not there when I started this…)