Those companies were mostly C#, with a bit of Ruby and JavaScript. The C# type checker generally made refactoring a non issue. In the case of JavaScript and Ruby, it was not much harder. Mostly find and replace. Sometimes a bit more work, depending on the complexity of the change. In general, the code bases were pretty well designed and modular enough that refactoring was pretty simple.

I tend to be a repl-driven developer, and I also use the product as I code. If you've ever watched Jonathan Blow programming on YouTube, that's similar to my process.

I find the majority of issues as I go. The bugs that slip through tend to be the surprising edge cases where I think I'm just modifying a single vertical, but there's some bleeding over into other verticals which I'm not using while developing. These sorts of bugs can usually caught by a handful of basic integration tests.