Does iCloud storage delete my original files after backing them up?

Files that are backed up are in original format , iCloud storage don't exactly delete the files after backing to iCloud however , if there isn't sufficient space in iCloud , users use optimise feature Optimise storage space on your Mac - Apple Support , so whenever a sufficient space is created those files will be downloaded .

you can uncheck the box of optimise , go to system preferences > in the bottom it will be shown Change iCloud preferences for your Apple ID on Mac - Apple Support

I don't use optimization so I'll leave you with these documents outlining how it works:

How to free up storage space on your Mac - https://support.apple.com/HT206996

https://support.apple.com/guide/photos/from-icloud-photos-phtf5e48489c/mac

Optimize Mac Storage: Stores smaller versions of your photos on your Mac when storage space is limited, and keeps the original, full-size photos in iCloud. Choose this option to conserve space on your Mac. To restore the originals to your Mac, just select “Download Originals to this Mac.”

Hello Triantafillia_Memisakis,

If you want to disable this feature, go to System Preferences and in your iCloud settings (Under Apple ID), disable the Optimise Mac Storage feature. This will keep your documents downloaded.

Hope this helps!

I see Apple has recommended different documentation but the ones I provided outlined the reason for what you were seeing with optimization in the context of photos. I just used those to illustrate the feature. (I guess neither you nor recommending team picked up on that.) It also talked about turning off optimization which can be applied to various categories.