This is how most software development worked before roughly the late 2000's. I remember working on a system that processed like a billion in revenue for a major corporation employing thousands of people, written in a mix of C and C++. Zero unit tests. They did have a couple of dedicated QA guys though!

That is insane. I would expect as many lines of unit tests as code. Doesn't ReactOS have over ten thousand tests.

> I wonder what they are doing instead that ensures the Kernel mostly works.

First, they do have unit tests (KUnit). However, I suspect the "real" tests that result in a mostly-working kernel are massive integration tests run independently by companies contributing to Linux. And, of course, actual users running rc and release kernels who report problems (which I suppose is not unlike a stochastic distributed integration testing system).