As a consultant, approaching new code bases is something that happens every day. As exciting as it sounds however, working on many different projects can be really tough. This is because in many occasions we are not provided with all the details that we need, but we just requested to fix something that is broken.
What makes these tasks difficult is that the data and code structures can be very large and not very intuitive. Also, it might not be clear who holds the knowledge on them, or in some cases the knowledge can be even lost; for example being held by people who left the company.