I wish I had never moved to iCloud

The wait time is not acceptable to get files to come back to the internal hard drive so they can be opened. The internal hard drive has filled up inexplicably since subscribing to iCloud, and files can't be copied off the internal drive it to non-iCloud external backups because there is not enough space.

I once turned off iCloud and that was a disaster. Some questions:

Is there a way to keep iCloud from moving files from my MacBook Pro and filling up the internal hard drive?

No. iCloud is a synchronization service, not offline storage. Anything you add to your Mac is in iCloud and vice versa. You can elect to optimize storage in which case your oldest/largest files may offload to iCloud only should your computer need space. This is an automatic and non user-controllable option. Optimized files are not resident on the Mac and need to be downloaded to be backed up using Time Machine or other backup software.

Can someone explain why the internal hard drive has filled up since going to iCloud?

All of your files are synchronizing

How can space be created so files can be moved off the internal hard drive?

You can move the files to an external drive or to an offline storage provider.