When you start work on a mature product you’ll probably start on a single part of the system. Hopefully this will be straightforward enough to understand and make changes on. No one part of the system stands alone though so at some point you’ll need to figure out the other parts and how they relate.

In an ideal world you could look at the existing, up to date, documentation and stop reading here. More likely there will be no documentation or it will be out of date and potentially misleading. This is a good time to ask around, figure out how things work, and document that knowledge for your future self and others.

Start at the high level, for example if you have a mobile app, a web app, some background services and a couple of databases then document the relationships between them first. You can go into more detail in separate diagrams that focus on one particular piece of functionality once you have the overall map in place.

The C4 Model is a good place to start if you haven’t done this before. There are templates for lots of different tools so you can use whichever one is handiest.

Could you find or draw an overall architecture diagram of the system you’re working on right now?