Some of these old discussions might help. Ignore the answers pointing to documention. I once had to dive into a medium size Java web application without documentation and it took me half a day to even find out where data was stored (in a H2 database) nevermind trying to install it or finding out what changed between the repository and what I found on the live server. Ultimately told the client it will be cheaper for them to find another contractor. Best of luck.

Ask HN: How to understand the large codebase of an open-source project? https://news.ycombinator.com/item?id=16299125

Ask HN: Studying large code bases https://news.ycombinator.com/item?id=9060832

Ask HN: How to dive into large codebase? https://news.ycombinator.com/item?id=12142807