This is regarding code bases [legacy or recent] that one inherits when joining a new company or a team and the original developers who built the app are no longer available for any reference. In this situation how would you wrap your head around the app and the code? How would you understand the business problem being solved and the various business rules that map to these problems or requirements?
IMO:<p>First step is to always be a user of the app and get used to the features of the app and if you are a software engineer, you will intuitively understand how the that feature is implemented.<p>Second, you can read tests for that project.<p>Third, if the app is for internal usage, you can talk to the customers directly on how they use it. If it is public facing, you can talk to your PM.