Understanding and reclaiming iPhone Storage

Keep in mind that iCloud is a syncing service, not a true archive. So yes, when you sync photos with iCloud, when you delete a photo on your iPhone it is also deleted in iCloud and anywhere else you sync photos (like to an iPad or MacBook).

But I now know what your issue is. 76 GB of system data is the issue. That is way out of line, which typically means you've got some corrupt data in System Data.

Here's how you can fix it - Backup your iPhone (preferably to a computer either Mac or Windows), then restore the phone from backup. You will gain a LOT of space which is being taken up by whatever has become corrupt by doing this.

This link will explain how to do that --> Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Open Settings > General > iPhone Storage and see what is using the most memory on your iPhone. By chance is it System Data? And yes, if you have thousands of photos on your phone, it could occupy that much space.

Also be aware that when you delete things like photos and files, they don't immediately delete from your phone. They go to a Recently Deleted folder for up to 30 days, so you can restore something if you deleted it by mistake. To fully delete things, you can open the Recently Deleted folder of the app they data was kept and permanently delete them, which will free up storage space.

I don't know how much memory your phone came with, but today it likely isn't enough. All of the newest iPhones Apple sells this year, come with 256 GB of storage out of the box and you can choose higher storage levels. Since you likely don't own a phone with enough storage to meet your needs, maybe it's time to consider buying a new iPhone with more storage?

Stephen Epstein wrote:

Removing Photos is tricky since there doesn't seem to be an indication of what's actually on my phone and what's in iCloud, and there are warnings that deleting photos will delete them from anything else connected to iCloud (I want to keep a "source of truth" on my local computer hard drive, which is backed up to two external drives locally and Backblaze offsite).

iCloud Photos tries to keep all synchronized photos on all synchronized devices, and in iCloud itself. The copies in iCloud are always full-size ones, but on devices where you have turned on "Optimize", the local copies may be reduced-quality, "space-saving" ones. (When to substitute them for full-size copies, or vice versa, is completely under the system's control.)

I have "Optimize" set on my iPhone, but not on my Mac, so that when I back up the drive that contains my system Photos library, I know that the backups will always include full-size copies of all of the photos in that library. Thus full-siize photos exist in iCloud, on my Mac, and in my Mac's local backups.