How does the 'Download and Keep Originals' function work?

I recently logged out of my Apple ID on my Ipad to do something and after logging back in, the photos from my iPad were in the iCloud and by that also on my IPhone. This happened before so I tried going doing what I remembered, Settings > Photos > disable iCloud photos. But what I didn't realize was that it was on "Optimize IPhone Storage" so it put all of my photos and videos from my IPhone in my iCloud backup. I looked up a tutorial on how to fix it and after a while my stuff was back but with the IPad pictures as well. This time I clicked on "Download and keep Originals" before disabling "ICloud Photos" but it also got rid of all my photos and videos. Luckily I still have some photos left but I had 1.500 and most of it just disappeared. I thought it must be in my iCloud backup but I've been waiting for the photos to load back in again and nothing has changed. And the GBs of my iCloud backup has also decreased for some reason. Could this mean that my photos have been permanently deleted?

Im not sure what I did wrong since all I tried was unsynchronizing my IPad and my IPhone. Everything worked perfect the last time I tried and I'm scared that I accidently made a big mistake and deleted many important pictures and videos from my phone.

I hope this isn't badly explained, I would really appreciate any suggestions or advice!