Yea, for me what really helped is once I interned at a hydraulic engineering company and I was tasked with debugging a custom software for pump modeling/optimization in GIS.

I had no background in C++ or the GIS I was dealing with and this was a one-off thing that was built for a project, but not maintained or documented at all.

The lead eng that wrote the software also had a long and detailed personal log of what he was working on. He just gave me that.

It helped a lot to understand not only how the software worked, but also the design decisions.