Sandboxed vs. Non-Sandboxed

On macOS, an app may or may not be sandboxed. Since macOS Sierra, apps distributed outside the Mac App Store can access iCloud APIs, so a non-sandboxed app can use these locations to sync data.

So, if you want to access a sandboxed iCloud location, you should go to the app container and use it as the home path.

$SANDBOXED_CONTAINER = ~/Library/Container/com.example.app/Data

iCloud Documents

Each app can have one or more document containers inside, named with its bundle ID:

~/Library/Mobile Documents/

The information and details of each container are stored here:

~/Library/Application Support/CloudDocs/session/containers

The info is stored in a plist file with the name of the container, for example:

iCloud.com.pvieito.Example.plist

And the icons are stored in a folder with the name of the container:

iCloud.com.pvieito.Example/40x40_iOS.png
iCloud.com.pvieito.Example/80x80_iOS.png
iCloud.com.pvieito.Example/120x120_iOS.png

Key-Value Store

This synced data is stored as a property list file in:

~/Library/SyncedPreferences/
$(SANDBOXED_CONTAINER)/Library/SyncedPreferences/

CloudKit

CloudKit works online, but macOS stores a cache of its contents in a folder appropriately called CloudKit:

~/Caches/CloudKit
$(SANDBOXED_CONTAINER)/CloudKit