iCloud Photo Sharing upload from apps has nothing to do with Photo Library on iCloud
iCloud Photo Sharing is a different service than iCloud Photo Library. iCloud Photo Sharing allows you to share an album with another iCloud user (friends, relatives...) while iCloud Photo Library is just a cloud-based storage provided by Apple.

Basically iCloud Photo Library allows you to store and organize images (either from camera roll or from apps)into  albums and folders so you can access them on all your devices connected to iCloud an even from the web at icloud.com. albums and folders you created can be found at the "albums" tab of the iPad photo app.

On the other side iCloud Photo Sharing allows you to create a shared album that you can share via a link or directly to an iCloud user. This album is displayed in the "Shared" tab of your iPad photo app. removing this album won't delete the photos it contains from your photo library.

Concerning Vectornator app you have two options to save your work on iCloud:


Save the artwork to your iCloud Photo Library (export to PNG or JPEG)
Save the artwork as a file on iCloud Drive (export as PDF or SVG)


Note that iCloud Photo Library purpose is to save and organize photos, not professional work or other artwork so you must not (and cannot) save SVG files to iCloud Photo Library.