Read the code

Documentation lies. Not on purpose. It just drifts. Someone writes a README, the code changes, nobody updates the README. Three months later a new developer reads the README and builds the wrong mental model.

Tests are better. Tests at least run. But tests can lie too. A test that mocks everything is a test that proves the mocks work, not the system. I've seen green test suites on code that was fundamentally broken.

The code doesn't lie. The code just is. It might be ugly, it might be confusing, but it's the truth. When I need to understand a system, I read the code. Starting from the entry point, following the happy path, then the sad paths. That's where the real architecture lives. Not in the diagrams.

Unless... you have something that is the code. But that's a story for another time.