Hi Jo. Yes, I think you are downloading or having a lot of files somewhere you don't know exactly where it is (but it seems it is not your iCloud drive).
Usually I have a lot of things on the desktop folder, documents, downloads, etc. Have you checked those folders? Please delete as many files as you can from there. Also, make sure you configure your browsers and apps to download stuff to the iCloud drive. For example, in Safari you should go to the preferences > General > Download location.
An app which is super useful is CleanMyMac X, I do not know if it has a trial version but you should definetely try it.