Universal Containers (UC) had been using change sets for deploying all of themodifications to its...

 In the change set model, the source of truth is a combination of the metadata already in the environment and the content of your change set, as you stated correctly. However, a change set cannot be retrieved from the developer’s workbench as a package, nor can it be pushed into the version control system. Change sets are only available within the Salesforce UI, and they are not compatible with version control systems. In the package development model, the source of truth is the metadata in the package project, which makes it easy to integrate to a version control system, as you stated correctly. Additionally, in the package development model, the best practice is to consider the dependencies and build them into the package so it can be deployed in any other target orgs. This ensures that the package is self-contained and does not rely on external components that may not exist in the target org.