iOS Ref

Design

• Resolution by device
• Display tech by device
• Camera tech by device
• Fonts by iOS version^Ext

OS by device

• iOS version by device
• iPadOS version by device
• watchOS version by device
• tvOS version by device

OS by usage

• iOS version usage

Hardware

• RAM & processor
• Hardware strings

Swift

• Swift versions^Ext
• Swift 5 cheatsheet^Ext
• All docs^Ext
• Swift Book^Ext

Swift guides

• Closures (mini)^Ext
• Functions^Ext
• Control flow^Ext
• Dispatch^Ext

Guides

• Entering recovery mode
• Objective-C block syntax

Tools

• Hex → Color/UIColor
• WTF, Auto Layout?^Ext
• NSDateFormatter^Ext
• API errors lookup^Ext

Xcode

• Xcode Releases^Ext
• Xcode Build Settings^Ext

Apple docs

• All docs^Ext
• HIG: iOS & watchOS^Ext

App Store

• Review Guidelines^Ext
• Marketing Guidelines^Ext

iosref.com, in "iOS Ref"