I’m currently working on two open-source projects: Fugu (https://fugu.lol), a privacy-friendly product analytics software, and Mapzy (https://mapzy.io), a store finder.
In addition to what others have said, go through the existing automated tests. If there's a good testing culture at the company, the tests should show you how the most important flows and functions work and what you can expect as input and output.
In addition to what others have said, go through the existing automated tests. If there's a good testing culture at the company, the tests should show you how the most important flows and functions work and what you can expect as input and output.