https://barosalki.tistory.com/entry/iOS-Swift-53-main-type기반의-프로그램-진입점
[what is main?] https://medium.com/@abedalkareemomreyh/what-is-main-in-swift-bc79fbee741c
[App Delegate?] https://dev200ok.blogspot.com/2020/05/ios-appdelegate.html