I can think of so many times this has happened before. People getting used to MVC, Dependency Injection, ORMs. Sometimes the code is so large you only work on a part of it, so you have to get used to a lot of not knowing.

So how do we build confidence anyhow when we can't know everything?

I think first, standard checks - does it compile? What do static analysis tools say - code quality, passes SAST analysis? Linters? Style checkers?

Do the tests run? I'm not comfortable with the tests I write let alone the AI.. by adding fault injection (fuzzing, chaos), and mutation testing, I have a better degree of confidence in the tests and code.

Spend even more time getting to know the product instead. Maybe I don't know the code as well as I used to, but I know the product backwards and forwards. That helps me have some new confidence that I didn't really have before. Knowing the greater whole.

Your comment made me think that people have been happily importing hundreds to thousands of dependencies into their projects that were written by some random guys on the internet, but they've just now started to become concerned they don't understand the code they're shipping.

> So how do we build confidence anyhow when we can't know everything?

Avoid known unreliable tools?