San Francisco font (and any other installed fonts) can be used in all Apple ecosystem apps (e.g. iPhone, iPad, Mac, Watch, TV, etc.) In fact Apple encourages developers to do so. Just don't download the font files themselves and embed them in your application bundle. Use system font APIs to get San Francisco font, such as UIFont.systemFont(size:) There are also UIFont APIs to load other installed fonts by name. However you are prohibited from using San Francisco font on non-Apple devices. Doug Hill https://github.com/djfitz/SFFontFeatures

You can use special tags to render web page text with San Francisco font. See here: https://developer.apple.com/forums/thread/127350?answerId=614912022#614912022 However, these use system-font tags which only render San Francisco font when it is the system font. So only Apple devices. Non-Apple devices cannot use San Francisco font.

If you're using a font in your app that is already installed on the system, you don't need any specific authorization to use them. As long as you use approved Apple APIs, like UIFont and NSFont to select them. However, you can't download Apple's installed fonts and embed them in your app bundle for distribution.

You should really not use this solution to load fonts in your app. For example if Apple ever changes the font names your app will break. Or if they change permissions on these folders/files so they are no longer readable. Instead use approved Apple APIs to load installed fonts. For example: UIFont.init?(name fontName: String, size fontSize: CGFloat) You can also check for how to install a custom font in your app: Adding a Custom Font to Your App