What is the difference between Copy Bundle Resources and Copy Files for Xcode build phases?

Copy Bundle Resources phase copies files that you want to be available in your bundle (.app). On the other hand Copy Files phase copies files to other (standard) locations accessible from your application (for example to /Library/Fonts) giving you also the option to copy them only when installing. You can also see relevant documentation here

Copy Files - Copies files from a project to an external specified location.

It is useful, for example, when you are creating an Objective-C Static Library where you should expose .modulemap^[About] or/and umbrella.h^[About] files

Copy Bundle Resources - Copies files that support your source code into internal structure. If you want to make sure the files you added will get copied to the application bundle

It is useful, when you are creating an app where you can add images and other resources

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