Universal Containers’ org is complex but well-organized in unlocked packages withtheir dependencies.

The best environment for developing a new feature in an org with unlocked packages is a scratch org with the package code that will be changed and its dependencies. A scratch org is a source-driven and disposable deployment of Salesforce code and metadata that is fully configurable and can be created quickly. A scratch org can also install unlocked packages and their dependencies, which can help ensure compatibility and functionality of the new feature. A Developer Pro sandbox is not a good option, as it is not source-driven and can have outdated or conflicting data and metadata.