This is an incredibly broad question, and the answers will probably be all over the place. If you are working on the front end, try to find the entry points and

This is an incredibly broad question, and the answers will probably be all over the place. If you are working on the front end, try to find the entry points and work from there through various hypothetical scenarios. Hopefully there is documentation on various parts of the system, or architects/devs who built it, but that is unfortunately not always the case… Find out the deployment process. Find out what to do if anything goes really, really wrong. Try to find patterns in the code. Just explore for a while and even try to create you own mental model of how things are structured.