Change macOS Catalyst App Bundle Identifier

On Catalina, Mac Catalyst bundle IDs are fixed with a auto generated prefix. See this WWDC 2019 session at 11:45

https://developer.apple.com/wwdc19/205

And this one at 44:00

https://developer.apple.com/wwdc19/235

but they both say effectively the same.

The behavior in the answer above was updated in Xcode 11.4. In 11.4, newly created mac catalyst apps will default to using the same bundle ID as iOS. If you want to enter a custom bundle ID, go to the signing and capability tab in your target editor and there will be a checkbox labeled "Use iOS Bundle Identifier"; once unchecked, you can easily create a custom bundle ID for mac catalyst.

Likely, many already configured their app's mac catalyst bundle id before Xcode 11.4, here is some documentation describing how to move over and how the new behavior works.

https://help.apple.com/xcode/mac/11.4/#/dev07ed024a6
https://developer.apple.com/support/universal-purchase/