My work in the past consisted mostly of dissecting and refactoring large legacy codebases. Usually after anybody who actually understood it already left and where the codebase is too large and rotten for anybody new to be able or willing to learn it.

I have my own process I apply for any change to code. The basic goal is to introduce zero defects to the code that I do not control/know. Which is crucial when I want to make thousands of changes to a huge application where if I make a mistake it can be very costly to the client, can cost me a huge amount of effort to fix and can completely undermine my effort.

Usually there is no good way to test the changes. I make hundreds or thousands of individual refactors, bundle them up and kind of hope I haven't broken anything in the process.

Some of these applications are just too large for one person to be able to comprehend. They either have vast user interfaces or vast network of integrations. That and no ability to test it in automated way.

This is the reasons why these applications usually aren't taken good care of -- new developers (new, not necessarily junior) are just too scared and unprepared to do anything than smallest changes and that with huge amount of effort.

From my experience, the programming language and environment plays enormous role in making this possible.

The most basic questions I may have when analysing the code I want to change are:

1) What is the thing I am looking at? For example, the exact type of variable, parameter, code for method, etc.

2) Where is the thing I am looking at referenced from? Usually when I want to modify a piece of badly written code, I have to be able to find and analyse every single usage of it.

3) When is the thing I am looking at being executed?

It is not just the ability to answer these questions -- I need to be able to answer them with absolute precision.

Since I program mostly Java, all that information is usually available to me. #3 is sometimes a little bit more tricky but if you can hunt all usages you can typically get to it with some effort.

But that is not the case for most programming languages these days. For example, I tried this with a large Python application and I have utterly failed and had to tell my client I can't help them. Which tells me that most programming languages are not suited to building large, complicated systems that somebody might want to be able to analyse and refactor in the future.