When your new at coding for macOS ( not using catalyst ). there will be a lot of serching for help online, and not a hole lot to be found.
- how to load a "splash screen" and after some time, go to the main view.
- replace the about in menu with a custom swiftUI view
- open a view in a new window or replace the view in the main window
the splashscreen is made thanks to https://www.raywenderlich.com
hope this will save you houres of searching online....