https://developer.apple.com/library/archive/navigation/
I arrived here via google search:
https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/OSX_Technology_Overview
Other conceptual sections I found via wayback machine:
https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFileSystem https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFrameworks https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPInternational https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPMultipleUsers https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPRuntimeConfig https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPSystemStartup https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/OSX_Technology_OverView https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/universal_binary