Why is my cloud storage full but no images synced?

I migrated my LR Classic catalogue containing 50k of images to Lightroom. It does show pretty much the same number in LR now so that seemed to work.

But how come it says 727.6 GB used from my 1TB storage but when I filter for "Synced and Backed up" there are only 80 images that come up? Is this number a virtual number indicating that this will be used once the upload is ready? Is there any way to see which files are actually in the cloud? It's only my 3rd back up but I still want to understand which files are saved there. (I deleted everything I had uploaded previously from LR via the web function to delete all files, so there was nothing on it before migration)

What is the actual difference between "sync" and "back up" in that case? I assumed sync means collection/album structure is synced with LR Classic and back up means the original is stored in the cloud?

I'm also very confused about the local storage, ie.:

Info for "Image X" shows in Sync Status:

- uploading (with the blue icon spinning)

- local: Original File

- Cloud: Pending

As a test I deleted Image X from my local drive - no change in LR, still says local. So is this actually linking to the file on my desktop or does it only mean it has been imported from local? Where does LR take the file from when it is NOT backed up in the cloud yet?

Any help is appreciated!

I have been using LR for almost 10 years now but this is all extremely confusing to me.

Thanks
Teresa