I've been working on an app that I started on a few months back with iOS 13 and now I'd like to have it only support iOS 14+ and also be able to build to a macOS app. I updated the checkboxes in the Project file to support macOS and then I created a new app file that includes @main (along with adding the appdelegate mentioned here https://www.hackingwithswift.com/quick-start/swiftui/how-to-add-an-appdelegate-to-a-swiftui-app) and I commented out "@UIApplicationMain" in AppDelegate.swift.

Now the build succeeds but instantly crashes for some symbol not found error with the directory mentioning maccatalyst (dyld: Symbol not found: _$s4Body7SwiftUI3AppPTl).

I created a new multiplatform app from scratch and noticed it has a new folder structure: Shared, iOS, macOS. Do I need to manually create this folder structure myself?

Did you ever figure something out for this issue? I just ran into the exact same thing. I could start over with a fresh multi-platform app, but I'm curious to see if you found a solution!