This is my situation right now on a contract I started a few months ago. A consultancy was hired and spent over a year building a system. All the in-house employees don’t really know how it works, it’s essentially a black box to them. So I’m digging through the code to make sense of it and no one from the business can give me test data that reflects what the system was meant to handle. Only thing I can really do is best guess and slowly learn how the business operates while at the same time fix bugs and launch new features.