When developers take shotcuts or decrease the quality to gain time, they will cause a technical debt. By doing nothing and letting the system age, the technical debt is growing too.

Independent IT Professional
When developers take shotcuts or decrease the quality to gain time, they will cause a technical debt. By doing nothing and letting the system age, the technical debt is growing too.
In a legacy system, what is it that you spend absolutely most time on? I would say, understanding code and finding out where to do a change. Most of us have been in the situation that we spend hours or days looking for exactly one single line that would be changed in a million line code base.
I recently heard about a project where the code was written in C#, but where they implemented the unit tests in F#. They did so because they wanted to learn and evaluate F# in a protected environment. I thought that sounded like a very good idea and wanted to try.
Dear Newbie, I know you struggle a bit with your new role and how to become a successful professional programmer. I think the best would be if I just told you, already now at start, what you need to know!
As a DDD (Domain-Driven Development) person, I like to be able to read the code as I read a book. This is of course very hard, but something that I strive for. Therefore I often use the builder pattern for unit tests.
When I've tried to learn more about functional programming, I've always felt that something is missing. There is a lot focus on the functions themselves, but less on where to put them in a structure.