What knowledge and skills will be useful for working with someone else's code?

Question

Programming

Someone creates software projects, but someone has to support them) I chose the second - there is always work, a minimum of competition, a large selection of projects.
Please tell me how you can pump yourself as much as possible for such work. What to read, what to study, etc.

Answer 1

Everything is the same as for working with your own code (everyone needs refactoring). As well as strong nerves.
Read books on refactoring, video lectures by Uncle Bob (Bob Martin)...