iMac fills OS X hard drive while copying files from an external hard drive to iCloud
https://www.youtube.com/watch?v=qeU2KFUFmbIWhy does my iMac fill up my OS X hard drive space when it is just copying files from an external hard drive to the iCloud. The video shows how the OS X hard drives memory is being fill. Before copying files, the "About This Mac" storage showed 370 GB was being used by iCloud and there was 52 GB of free space. After copying 14 GB from an external hard drive (Not OS X hard drive) to iCloud, it showed that the iCloud was using all 500 GB of space and there was only 37 GB of free space. It seems my OS X hard drive is being used like temporary storage for the file transfer, but it does not remove the files after the copying is done. Two of my iMacs (2017 and 2019) running Big Sur 11.1 now have their OS X hard drives filled. Anyone know how to get the OS X hard drive space back, so I can use my computers? Settings issue?