Apple Design Site
https://developer.apple.com/design/
Apple Human Interface Guidelines
Get in-depth information and UI resources for designing great apps that integrate seamlessly with Apple platforms.
https://developer.apple.com/ios/human-interface-guidelines/overview/themes/
Google Material Design
Material Design is a unified system that combines theory, resources, and tools for crafting digital experiences.
Material Design Guidelines
We challenged ourselves to create a visual language for our users that synthesizes the classic principles of good design with the innovation and possibility of technology and science. This is material design. This spec is a living document that will be updated as we continue to develop the tenets and specifics of material design.
https://material.io/guidelines/
