how can we free space on mac by moving files to icloud

iCloud is not a storage or backup service. It is designed for cross platform sharing. It mirrors what is on your Mac. You need to get an external disk to store that data on, then you will have it securely stored. Also, if you don't have a separate dedicated external disk for backups, eventually your HD will fail and your data will be lost.

Move user data to an external drive. If important, backup that moved data too.

Well, thank you, Ronasara! That certainly answers my question. I've been trying to figure out how to free up disc space on my Mac hard drive. Looks like I will have to offload more files to DropBox. You'd think that Apple would prefer we use Apple's iCloud to free up disc space instead. Guess they want to sell bigger hard drives... Thanks again, Ronasara! You've been very helpful.

@ku4hx Thank you! I spoke with Apple Support and they suggested also a network accessible server (NAS) connected to the internet, a good brand like Seagate, as a cloud-based alternative that I control. In our case, as full-time RVers, we are mobile, so I will need to see who I can ask to host it in their home or business, someone I trust, with excellent internet speed and up-time. In the meanwhile, I will use an external SSD hard drive that I set to back up to DropBox. The trouble with Dropbox is that very old (15+ years) audio and presumably video files are getting somewhat corrupted over time. I am going to contact DropBox about this problem. The external SSD or NAS are solutions for now. The other issue with DropBox is that deeply nested files (files within a series of folders) are no longer going to sync due to the path character limits they are using in their new set up for Mac-based computers. I sure wish Apple would take notice and solve this problem for us with off-line options for its iCloud storage! Thank you for your suggestion!