When you delete a photo, it warns you that the photo will be deleted from all your devices. However, there's a built in album called "Recently Deleted", and all deleted photos stay there for 30 days until being permanently deleted. Every thumbnail within that album shows how many days remaining until deletion.
That's exactly how it works. EVERYTHING is synced, and in near realtime too. What I love most about it, is all the built in 'albums' that are always synced - Photos, videos, Bursts, Panoramas, Slo-mos, Timelapse. It's great being able to quickly see all slo-mo videos taken for example. But yes, all Albums are fully synced. I've gotten into the habit, when at an event taking a bunch of photos, I'll grab them all, dump them into an album on my phone, delete unnecessary duplicates or bad shots, and it'll all be synced back everywhere.
Apple is using some sort of formula here (when choosing optimize storage instead of download full copies). They are definitely being conservative from what it seems, and not just haphazardly filling up unused storage.
For ex, as I mentioned in my previous post. My iCloud photo library is 170GB, and on my 128GB iPhone it's using 21GB of space for photos - this is mainly because ~18-19GB of that were shots taken with my phone over the past 6 months with iCloud Photo library turned on. If storage space were to become an issue on the phone, local copies of photos would be quietly removed, leaving optimized thumbnails instead.