https://techcrunch.com/2019/06/03/ios-apps-will-run-on-macos-with-project-catalyst/

Apple announced during its WWDC keynote that iOS apps will run on macOS, starting with the next major release of macOS this fall — macOS Catalina. Third-party developers will be able to release their iOS apps on the Mac starting this fall.

This might seem like a small change, but it requires a ton of radical changes behind the scene. Last year, Apple teased this move by porting Voice Memos, Apple News, Stocks and Home to macOS.