They also do a lot of tech blogging on their Open source code, especially the Safari and WebKit team have some excellent posts regularly.
Sure they have proprietary magic in there but it is not as big a part of the pie as people imagine, and certainly less than in years past.
The same is true for Microsoft, who now famously aims to be the biggest Open Source company in the world (having acquired GitHub, Xamarin and many others, as well as made partners and friends out of enemies of the past, to help them on that journey).
In fact I can’t think of a single major company in the business which hasn’t embraced Open Source to some degree. I don’t think that the Apple strategy of controlling the entire experience means what it used to anymore. It doesn’t mean locking you in to just one way of doing things, we now know that we need the compiler, tools and stack to be available and truly free, as a bare minimum for this to work and experience shows that the more work we share, in general, the better an experience we can present to users and developer.
Open Source has won, all these companies taking on designing their own chips, datacenters, OSes, languages and so on, they would not be possible without that commonly shared mass of work.
Famously FaceTime was supposed to be an open standard, until someone threatened to sue them for damages to the tunes of X times infinity, which is a fairly large dollar amount for any given value of X.