Header files are not targets; only implementation files. Header files are included indirectly via the implementation files, which trigger compilation. You don't even have to include the header files in the project navigator, but it is super convenient. Just think about the standard headers or Apple's supplied headers. None of them are included in the project navigator or are included as "targets".

Look at the preprocessor output. Not only does it show you exactly what the compiler sees, but it also shows you the complete pathnames Xcode is using for each header, including all the nested headers.

Click on an implementation file (.m, c) and then select the menu item, Product: Perform Action: Preprocess "filename".

Glad to see you making progress and, hopefully, sticking with software development. :) I'm not sure if you are using source control, but it's free and easy to set up in Xcode. Any accidental text changes to your files would show up instantly.