How to tell macOS to move things to the cloud and free space on my laptop? I use iCloud Drive and I see sometimes macOS keep files on the cloud and locally on the laptop there is only a link to the file. Whenever I need the file just click the link and it downloads. So far so good.

I have a lot of files, so no possible to keep everything locally. And I would like to keep adding files and take advantage of macOS functionality. I added many files (cd/dvd iso images) and disk is almost full. I guess at some point macOS will decide to keep files only on the cloud and free some space locally. Can I force this? How?

I have a lot of free space on iCloud Drive, like 5 times size of laptop ssd.
iCloud is a syncing solution and not a backup in the same sense as Dropbox or OneDrive.  It's not possible to keep something on iCloud and not on a device at the same time. 

The setting Optimise Mac Storage setting creates a reference to the documents when your Mac is low on storage but still keeps the file in sync.  The file is not really removed from your device because if you delete the referenced file on the Mac it will delete it in iCloud, too, which is not what you wanted. It runs on its own schedule and constraints. 

What iCloud offers is the ability to access files from all devices that are signed-in using that same Apple ID.  It was never meant to be, or designed, to act as an offsite storage.

If you need to do that you'll have to look at using other providers, like the one's I've already mentioned, for those file.