Maybe we need a definition of “understanding how software works”. There’s the technical aspect (computation theory, computer organization, compilation, executable format, …) and there’s the necessity aspect (the domain).

The technical aspect can be learned although you can stop at the top of the abstraction tower (the programming language and its ecosystem). The domain aspect encompasses the whole world pretty much. Contributing to Blender does not qualify you to review a Krita patch. You have to learn the latter’s code first.