[funwie]

In addition to already mentioned steps,

0) Read the code base docs (or README).

1) Pair with someone with knowledge of the code base or ask them to walk you through the code base.

2) Identify the public interface to interact with the app/api. How do consumers use the software. Play around with the app or api to get a sense of how things link up.

3) Identify various tools used in the code base(db, messaging, external api, etc). Now you know each tool is setup somewhere and used in one or more places.

4) Identify the patterns and conventions used (CQRS, mediator, dependency injection, middleware, pipelines, logging, etc). Now map the flow of each public interface using this knowledge.