Software development process at Apple

I was always curious what the development process is at Apple. I know there are many different projects and teams and they all have different processes. Some are agile, some are waterfall, TDD, BDD, etc. I am more curious about how some teams go about writing code that is of such good quality. How much unit testing are the developers writing, how much integration testing, who writes the e2e tests, how many defects are being caught by QA and what's the process around developers introducing bugs in their check-ins that they have not caught themselves. What code coverage percentage is an acceptable number? Another stat I am interested in, how much time does a developer spend on writing tests as a percentage of working on their feature? 50%, more, less?