Why sync is not a backup
A backup is a separate copy that survives what happens to the original. Sync is the opposite by design: its whole job is to make every copy identical, including the mistakes. Delete a photo, empty Recently Deleted, and it is gone from every device at once, because that is what you asked for.
The 30-day window in Recently Deleted is the only thing standing between a wrong tap and a permanent loss — and it syncs too, so recovering on any one device recovers everywhere.
What it costs you in storage
Your whole library counts against your iCloud plan. The free tier is 5 GB, which a phone camera fills in a matter of months, so anyone with a real library is on a paid plan. What it costs on the device depends on a separate setting — Optimize iPhone Storage — not on iCloud Photos itself.
This is also the reason a single cleanup pays twice: clearing real clutter shrinks the library everywhere it lives, on the phone and on the plan.
What you can do about it in iOS
- Switch it on or off: Settings › [your name] › iCloud › Photos, or the iCloud section of Settings › Photos.
- See what it is using: Settings › [your name] › iCloud lists the plan and what is filling it.
- Make a real backup first: before any large deleting session, get an independent copy onto a computer or another service — one that a sync cannot reach.
Note that iCloud Backup is a different feature, and it does not separately back up a library that is already in iCloud Photos.
What this means for cleaning
Everything you clear travels. That is fine when the 30-day net is real, and it is why Kept routes every delete through Apple’s own Recently Deleted rather than removing anything itself — the recovery path is the one Apple already gives you, on every device.