Where can we find Apple Technical White Papers?
Developer Documentation

Apple publish their technical documentation, including occasional white papers, on the sub-domain developer.apple.com.

Apple currently promotes https://developer.apple.com/documentation as the official source of technical information about their products.

Archive

The archived documentation contains many useful documents. These are marked as no longer being updated, but their contents are accurate at the date of publishing.

Of particular interest will be the technical notes. These documents contain a wealth of information, such as the excellent TN2083: Daemons and Agents and TN2124 Mac OS X Debugging Magic.