This thread makes me think of when I was refactoring a COBOL application when I just started my SWE career in 2016/2017. The original program was from 1989 and things like git blame, ticket systems etc. wasn't there. There was rudimentary version control, but that didn't go back to far, and also surprisingly there was some documentation. But the major thing that helped me answer the "why's" was that the original programmer still worked in the same department, and he actually remembered some of the choices he made.
Other than that, it was a matter of remaking the same mistakes and figuring out the "why's" the hard way.