When your Mac is low on storage, only recently opened messages and attachments are stored on your Mac.
Messages: iCloud stores all your messages and attachments.To download the original version of the photo or video, simply click it to open the file. When your Mac is low on storage, only space-saving versions of your photos are stored on your Mac. Photos: iCloud stores all of your photos and videos in their original, full-resolution form.
To access files stored in iCloud, simply double-click the icon to download the file. Files stored in iCloud have a download icon (a cloud with an arrow pointing down). When your Mac is low on storage, only recently opened files are stored on your Mac. When storing files on iCloud, you can choose from one of three options: Storing your content on iCloud, Apple's cloud storage service, helps you save space on your computer while still having easy access to your files.
