For anyone who wants to learn about working with and on a "legacy" code-base, check out Michael Feather's book "Working Effectively with Legacy Code"

In the beginning it goes over code smells and how to find refacing seams in your language of choice (C, C++, or Java) and then each chapter is a group of techniques you can use.

Working on a service that had both a very old monolith and some brand new microservices, I found it invaluable. I think the first lesson I applied from it was using pinning tests for safer refactoring.