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.
I'm a fan of Open Source and have a growing interest in serverless and edge computing. I'm not a big fan of spiders, but they're doing good work eating bugs. I also stream on Twitch.
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.