I am not overly optimistic, but if you are able to run the servers locally and run the frontends locally and have the list of all features the application has,

I am not overly optimistic, but if you are able to run the servers locally and run the frontends locally and have the list of all features the application has, then you should be able to execute them all locally and underatand how it works. It will not be fast, but if you are senior, and the technologies are known to you, it should not take extremely long.