Problem exporting photos from iPhoto. Problems with date created, etc

Hello,

Recently I've decided to create a new backup of all the photos I have on iCloud to a hard drive and another cloud services, due to personal reasons.

Exporting 25 thousand photos has raised many issues. The main issue I have right now is the date created. I do understand what Terence Devlin mentioned

"There are two kinds of metadata involved when you consider jpeg or other image files. One is the file data. This is what the Finder shows. This tells you nothing about the contents of the file, just the File itself. The problem with File metadata is that it can easily change as the file is moved from place to place or exported. Emailed, uploaded etc."

But the problem is still there, and it is strange. So I exported, (also, drag and drop, export original) an image taken in January (iPhoto recognize it is taken in January.)

Opened it in Preview, the created date shown is wrong. Why? I uploaded the same file to Flickr, Google Photos, none of them can't recognize the actual creation date.

Import the exported file with the wrong creation date shown(in Preview) back to iPhotos, the creation date is back to the actual date in iPhotos. Why?

Is it because of the camera used? Exporting images taken with a Nikon camera does retain the date created. Or is Apple messing with me so I won't be able to switch to other platforms? I believe exporting photos from iPhotos is becoming harder and harder. Currently, I have limited storage on my Macbook Pro, exporting these images (to an external hard drive) still requiring my Macbook storage, making it hard to export in a huge amount. Errors are popping out very often while exporting (such as loss of connection, even though my connection is stable). Apple recent decision to no to let us select multiple images on iCloud on the browser is making it harder.

Currently using an iPhone SE 16GB, it is impossible to have my iCloud photos turned on(with optimized space) to view all my photos. Just image thumbnail is taking too much space.

I hope someone would be able to help me with this, much appreciated. Thank you.

Kind regards,

Sean