I have read online, from multiple sources, that icloud drive is like Google drive or Dropbox, so why would iCloud drive take up 58 GB on my internal drive. I thought putting files in the iCloud drive was storing them in the cloud. Google drive and Dropbox doesn't take up a lot of space on my internal drive. Why is iCloud Drive using so much space on my internal drive?
Depending on settings, for both Dropbox and iCloud the files may continue to take up space on the originating computer, and on any computer that has been "synced" to allow offline usage of the files.
For iCloud you control this behavior by the storage preferences:
Open "About This Mac" from the Apple () menu, then click Storage. There is an overview of your free space, space used by different types of files, and options for optimizing usage.
iCloud is different from Google and Dropbox in that it's not primarily an external storage; it's a syncing solution.
This means that files you put in iCloud are available and accessible from every device that’s signed in from that same Apple ID.
To do this, it has to keep a purgeable copy of your data in each device. It will periodically sync changes to the files across all your devices.
So you can use iCloud like you use Dropbox and Google but those service have a fundamentally different purpose than iCloud.
MacWorld says (and I seem to recall hearing the same) that the files stored in iCloud on your local drive are essentially a local cache and as such generally purgeable if you start running out of space on your local drive.
Basically it is there to give you faster access to some files, and on the chance that you need one of those files when you have no network.