The short answer
With iCloud Photos enabled, deleting a photo also deletes it from iCloud and your other synced devices.
Decide what you actually want before touching Delete: more free space, a quieter photo library, or an independent copy. Those are different jobs.
Free up space without deleting your library
In Settings → your name → iCloud → Photos, Optimize iPhone Storage keeps full-resolution originals in iCloud and smaller local copies when needed.
This is the first option to consider for a full phone. Check available iCloud space and upload status before relying on the cloud copy. It does not move a photo into a private vault.
Turning off sync is a separate decision
Turning off Sync this iPhone stops subsequent photo changes syncing with that device. Apple offers download or removal choices during this process.
After verifying that uploads have finished and the originals are available in iCloud, you can turn off syncing for this device and choose Remove from iPhone when offered. This removes the synced copies from that device; it does not delete the iCloud library. Do not choose the account-wide Turn Off and Delete Photos option.
Read each confirmation carefully. Turning off syncing for one device and deleting a cloud library are different actions. Do not experiment with your only copy; use Apple’s instructions linked below.
When you want a separate private collection
Veil imports a separate encrypted copy, opened with your pattern. A vault still uses your iPhone’s storage, so moving files into it is not a storage-expansion trick.
Start with disposable samples. Open imported photos and play videos, save an encrypted backup, and test recovery with its matching code before removing originals. Keep that backup outside this phone. If you later remove originals in Photos, the iCloud Photos deletion rule still applies.
Already deleted something?
Apple normally keeps deleted photos in Recently Deleted for 30 days. Recover them there before that window ends.
Then choose your next step based on the goal: optimize storage for space, use the Hidden album for everyday browsing, or maintain a separate vault and recovery copy for independent access.