Start with the Interfaces, the subsystem boundaries will tell you more about the system than most other parts. I usually pick a few of the biggest ones or ones

Start with the Interfaces, the subsystem boundaries will tell you more about the system than most other parts. I usually pick a few of the biggest ones or ones related to the 'easy' task that I've been assigned as the new guy. Give each one a page in your notebook and start making notes. Go to their implementations and note what other interfaces they communicate with, check the tests for those interfaces and how they're Mocked. Make special notes when the implementations interact with external systems (other servers, web apis, databases, etc) keep going gradually till you've mapped enough of the system that you feel comfortable