I'm confused too, Gali, sorry. This reply has been meant for Foxfifth in a completely different discussion. I have been searching for my reply and could not find it again.
For some unknown reason, Apple has decided to turn on iCloud Photo Library by default every time you sign out of iCloud and then back in again, so 2500 or so of my photos, and a dozen or so of my albums uploaded, but that's all that is up there. (I've sent the Photos team feedback about this).
Yes, there is an abundance of posts by OPs who have been using iCloud Photo Library without knowing it. This default is ridiculous and dangerous. iCloud Photo Library should only be enabled, if the users are made aware of it and understand how it works.
And even worse is that "Optimize Storage" will be enabled by default on iOS devices. And this is deliberate. I could not believe it, when I saw it in the iPhone user guide.
http://help.apple.com/iphone/11/#/iph961b96c4d
Optimize your storage or keep all your photos and videos in full-resolution on iPhone. Optimize iPhone Storage is on by default. It manages space on your iPhone by automatically keeping your full-resolution photos and videos in iCloud and smaller versions optimized for iPhone, as needed. To keep the full-resolution originals on your iPhone, go to Settings > [your name] > iCloud > Photos, then select Download and Keep Originals. Your originals are always stored in iCloud.
Without alerting us, Photos may be removing gigabytes of data from our iPhone and we may not be able to restore them for a long time, if we are away from a decent Wi-Fi or do not want to pay roaming fees while traveling.