How to understand the received project?

Question

ASP.NET

Good day.
Recently I changed the project and got into an unpleasant situation - I need to implement the task, but how:

• The business task is partly clear, but how to implement it in code is not clear.
  
• There is no documentation for the project, everything is in the minds of other programmers (more precisely, one, and for the most part his answers do not help, because they are superficial or do not work, .. you can understand him - he has his own tasks)
  
• My task requires generating data based on the previous generation stage (which does not work correctly, which means that there is either no data at all, or they are partial and it’s scary to be based on them)
  
• Generating data in the head and inserting it into the database is not an option, the database contains its own processing logic, which is not always clear and obvious; not the fact that the implemented solution will, in principle, work on these data
  

Although I have about 5 years of development, I worked on 10 projects at different stages of development of these projects, but this time panic and fear --- the day of investment, I didn’t understand the conclusions, I didn’t understand, etc. ". Usually, there used to be documents on the project, BA, or QA that could tell what and how even in the database it should be saved.
How do you usually understand a new project that has been written for 3-5 years before you?