Empty directory this cmake files

Hi all!
using doc. v 1.9.0
need help this ’ Writing your First Application’
in section ‘Build the Application/Firmware’ some instruction

The application is now complete. In order to run it you first need to make sure that it is built as part of PX4. Applications are added to the build/firmware in the appropriate board-level cmake file for your target:

but my folder is empty
before that, everything went well