How to understand complex systems?

If you come across a system you need to understand, where do you start? How do you fix a bug or add some value with a new feature?

TLDR: A few tips for faster onboarding and less stress (hopefully).

A blog post where I describe the steps that I use to get up and running in the shortest possible time.

(Why a link to my blog? I like to keep control over the content I publish and a central place for future updates. This way both ends stay relevant)