> What keeps the software alive are the programmers who have an accurate mental model (theory) of how it is built and works. That mental model can only be learned by having worked on the project while it grew or by working alongside somebody who did, who can help you absorb the theory.

That's the easiest way to learn it, but it's not the only way. After a while, it even becomes fun to read through other projects (some of your dependencies, e.g.) to see how easy it is to develop this 'theory' (even if it is fleeting; in the case of dependencies, you'll at least keep a rough idea of the cost of that dependency, relative to it's value)