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.
Information about new tools and updates such as the latest about Swift, and SwiftUI can be easily found. And testing your apps with the help of tools such as TestFlight makes things so much easier.