Navigating preexisting app: Best practices and tips?

Certified Associate Developer

I've been hired on as a developer for a preexisting app with a large codebase.  Some of the developers who have created the original interfaces have left the project and it's difficult to navigate the app with sample data because it interferes with data that the team is using. Does anyone have any tips on how to find relevant objects and otherwise navigate the app via code.  Granted, tips on how to navigate as a user and then find the objects in the codebase would also be helpful.

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