Does Apple use containers on a Mac?
This container isn’t a ‘unit of software’ (like a Docker container, don’t get your hopes up!) but rather a container of data used for cloud synchronisation.

Other containers for sandboxing data exist at
~/Library/Containers and ~/Library/Group Containers:

Apple documentation of data containers and sandboxing exists at


https://developer.apple.com/documentation/cloudkit/ckcontainer.
https://developer.apple.com/library/archive/documentation/Security/Conceptual/AppSandboxDesignGuide/AppSandboxInDepth/AppSandboxInDepth.html