Analyst and legacy: how to reconstruct the structure of an old system?

A frequent task for an analyst is to reconstruct the structure and way of working of an existing system. This is necessary if the customer wants to replace it while maintaining the functionality, or if we are building a new system that must be tightly integrated with the existing one. Usually, the documentation for the existing system and descriptions of business processes are usually outdated, they do not correspond to each other and the real situation.

I will share my experience in solving this problem using several cases: how to understand the architecture of the system and its internal structure, as well as how to integrate the system into the business architecture.