Reference book or websites on how Mac OS works I'm looking for a good book or other set of reference materials on how MacOS works.  I'm not concerned with the GUI tools or advanced Finder tips or things like that.  Rather, I'm interested in the UNIX underpinnings.

I primarily used Linux for several years and still have a few Linux machines around, and much of my Mac use is in a Terminal.  But I have a basic grip on how a Linux machine works that I just don't have for a Mac.  For instance I know most Linux distributions use systemd, so I can poke around in that documentation.  I know the Mac uses launchd.  What would be nice is a book that explains these sort of technologies.  Other examples would be how application bundles work, or how kexts work.

The rough equivalent of "How Linux Works" from No Starch Press for the Mac would be good, but I can't find any books like that.  Even some decent webpages would be good - I can't find any on Apple's site.  Right now the best I can do is rummage around through the manpages on my system.
https://developer.apple.com has a lot of information, basic registration is free.
The books from http://newosxbook.com/index.php have more technical content than most macOS users/developers need in their lifetime.


Besides that you should be able find a lot of information by just googling for "macos launchd getting started" and similar search terms. For specific questions about problems in getting something done there is also always this site here :-)