In case you wonder how to delete system data on mac, note that System Data is not a specific folder which one can just delete. Rather, go over the storage of your Mac via System Settings > General > Storage and see which files can be deleted. These could include such as files that you no longer want to keep. You may delete the downloads, applications which you are no longer using, device backups, large files and other things that you don’t need any longer. Deleting files in the Trash will free up even more space. There are some caches and temporary files that are controlled by macOS itself.