Apple Developer Support
It is great to see all the support that Apple Developers get in terms of tools, ecosystem, community and more.
For starters the Developer Support portal has a ton of information for the new comer as well as for the more expert of experts. Including guides and documentation for tools such as Xcode as well as information for developing software for MacOS and iOS.
Information about Design is available in the same place, including Human Interface Guidelines, Fonts (including downloads for San Francisco!) and information about accessibility and localisation.