Mac Catalyst

Mac Catalyst (codenamed Marzipan, iOSMac, and UIKit on macOS) is a component of macOS that bridges iOS app frameworks such as UIKit such that they can run on a Mac.

Initially previewed in macOS Mojave with ports of the iOS News, Stocks, Home, and Voice Memos apps, it was rolled out for use by all developers in macOS Catalina. Catalina introduces Find My and Podcasts Catalyst apps.

In macOS Big Sur, an "Optimized for Mac" mode was introduced, allowing Catalyst apps to use macOS's native UI controls. By contrast, apps "Designed for iPad" use an appearance near-identical to iOS, and have the entire user interface scaled down to 77%, creating some blurriness of text and graphics. Big Sur replaces Maps and Messages with Catalyst apps.

macOS Monterey replaces Books with a Catalyst app.

macOS Ventura introduces Weather and Clock Catalyst apps.

On Apple Silicon Macs, apps from the iOS App Store can be installed directly from the macOS App Store. App developers must opt into this feature. iOS App Installer.app in /System/Library/CoreServices can install appropriately signed .ipa files. Apple has changed this feature several times, changing whether sideloaded apps (ones signed by a development certificate, rather than by the App Store) are or are not supported. At present, apps must be signed by the App Store to be installed. By contrast to Catalyst apps, which are Mac apps built for the  platform, iOS apps are fully unmodified - they are wrapped with a helper app that launches them.

A mirrored structure of /System/Library is installed at /System/iOSSupport, containing all iOS frameworks (some of which are stubs), and copies of frameworks that override macOS ones.