I am not sure if there are many people on the forum that have Zorin on Mac, so you may need to be patient to wait for a valid reply. Most like myself run Zorin on a PC, are less qualified to help you than Mac users.
You can try a websearch "Mac command key instead of ctrl ubuntu 18.04". I have just done that to see what comes up. Zorin 15.3 is based on Ubuntu 18.04. This is one search result that may (or may not) help you.