On MacOS, applications are a package, a special type of folder where all the files required for them to work are included.
Is there any way to place files (for example, text files) inside a Xojo project so that they are automatically placed inside the application package?
And, if possible, is it possible to define the folder hierarchy inside the application package “folder” that will be created when the project is built?
Investigate the Build Steps feature within the IDE.
To supplement that great advice, the documentation:
https://documentation.xojo.com/topics/build_automation/introduction.html#Copy_Files
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Thank you all 
Drag your file nto the Navigation Pane.
You can update that file wthout the need to do anything inside Xojo…
You can also use App Wrapper 4 to add files to your application bundle during development or distribution.
It can be set to only add files for a channel (app Store or Web) and to use hardlinking, which minimizes SSD wear while improving build time performance.