Over the last month or two, Apple has at last been improving its user documentation. Although these online manuals aren’t as comprehensive or detailed as we might wish, they are a step in the right direction.
The macOS User Guide provides broad coverage of macOS 10.14 and 10.15 (switchable between the two on most of its pages).
The iCloud User Guide gives quite comprehensive coverage of the features available at iCloud.com, which are explained for macOS, iOS, iPadOS and Windows.
Apple Platform Security goes into more specific detail on security features, across Apple hardware, system software, apps, and services. This contains more technical information. If you want to view or save a PDF of this, there’s a link at the foot of that page.
The Deployment Reference for Mac is aimed at system administrators, and covers provisioning, configuration and identity management, and content distribution.
The Visual Index to the macOS Human Interface Guidelines is a mine of useful information about the user interface, for both users and developers. If you don’t know the difference between a Popover and a Sheet, this is the place to discover.
For developers and advanced users, Apple’s overview of AppKit has links which explain many of its components, and is another excellent starting point for understanding the internals of most apps.
Apple’s Design and Development videos include not only those from WWDC, but more recent Tech Talks covering the latest technology. Many of these are valuable for advanced users too.
Developer documentation about notarization starts here, which links to other related topics.
Apple provides a full list of all current exchange and repair extension programmes.
Apple’s main System Status page, covering iCloud and other user services, is always worth checking if you are experiencing problems with any of them. There’s a separate page covering developer services such as the Apple Notary Service.