I scan everything that is closely or remotely related to the code base: the code, commit logs, diagrams, bug reports, change requests, user manuals, tests, tech

I scan everything that is closely or remotely related to the code base: the code, commit logs, diagrams, bug reports, change requests, user manuals, tests, technical logs, databases, other storage, cloud infra, ... Usually at least one of them stands out, so I at least read this through (usually diagonally). I might also pick different things based on my goals. Once I think I have a grasp of the high level aspects, I start pairing or validate with tiny feedback loops. Update: I also create my own (naive) helicopter view diagram of the context and validate it with people on different levels.