How to get long movie clips from iCloud to Google Drive
You have to use your computer as an intermediate step.
If you open 2 windows, one open to iCloud Drive, the other to Google Drive, then you can drag from one to the other … but all the files will go via your computer to get from one to the other. Depending on available space you may have to do it in stages.
If you're working from web access for both, then you will have to download, then upload, as two separate steps for each file.