Mouse support is not a standard feature, but is instead available as an AssistiveTouch option within the Accessibility settings on your iOS device. According to developer Steve Troughton-Smith, who discovered the feature, it also works with the Apple Magic Trackpad.
As with other AssistiveTouch features, the mouse cursor on the display looks similar to the touch target normally in iOS, simulating a finger touch with a mouse instead.
Though this is an Accessibility option at the current time and may not have the best user experience compared to touch, Apple could further implement mouse support in future updates, making it a more mainstream option.
iPadOS, the operating system that runs on the iPad, also has many other new features like updates to multitasking, improved gestures, and more, and going forward, it will be independent of iOS, though it still includes all of the iOS 13 features.