It's a bit of both. The D:OS Mac "installer" is a simple extractable DMG file. As with most Mac apps, you just mount (double click) the DMG file and then drag the APP file to your Applications folder (or wherever you want it).

However, the patches are executable PKG files, and they're coded to write patch data to "/Applications/Divinity Original Sin.app". Depending on how you've set up permissions on your machine, you may need to enter your admin password to apply any patches.

On a side note, in case you have other permissions concerns, user game data is not stored in the APP, as is often the case in other Mac applications.

User data, including save games, is stored on a per user basis in your "/Users/<username>/Documents/Larian Studios" folder.