> But when it comes to writing tests and designing my code (OO or FP) I still am not sure what the best way to go about it is. There is so much contradictory and sometimes just plain confusing advice.

Well, programming is not a monolith. Different high-level requirements demand different standards and processes, even though for many things a standard solution works. A safety-critical product (or just one that can lose lots of money) demands a different process than a toy app.