How to get more space on my hard drive [iMac 2T]

I have a 2017 iMac with 2T drive split into two volumes [Mojave and Monterey-I'm still moving things from Aperture]. I have tried to delete what I can from each volume so there is more room on it, but unsuccessfully. Part of the problem apparently is that Mobile Documents in User Library has 935GB in it =-= I have the bundle 2T of iCloud space and looking at the iCloud Drive itself it's only using 440GB so not sure what is going on. How do I have things left in iCloud but NOT downloaded to the various devices == I mainly want to access wherever I am, not have the stuff also on various drives. I have moved items off the drive where possible and also run TidyUp to clean out dupes but there is still only 240GB of space left --