[tomduncalf]

On a practical level, getting really familiar with grep/similar for searching the codebase and using a debugger (I like the one built into VS Code for working with Node) will help you when trying to work out how it all fits together.

Depending on time and other constraints, upgrading the codebase to typescript would be a great way to both familiarise yourself with it and to make working with it more productive, but obviously you’d need client buy in.