Hi Dave Kennard,
Welcome to the Apple (user-to-user) Community.
Re: How to save GarageBand Project without files
Am no expert in this area, yet your request surely deserves some response.
Seems a project needs the audio files in the same "package" in order to find them if you re-open the project.
A project is the document that contains all your recordings, the location of media files you add, and all the changes you make.
For saving space: it may work well to have the original files with the project, and "shortcuts" to those files saved where convenient.
Apple Support has articles and User Guides online for how the finer details of projects are set up to work.
Create and save projects in GarageBand on Mac - Apple Support
Intro to sharing in GarageBand on Mac - Apple Support
macOS app user guides: GarageBand for MacUser Guide - Apple Support
More help / info / support / contact options etc:
Garageband Support Website:
GarageBand – Official Apple Support
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How to: Save Logic Pro projects - Apple Support
Logic Pro X User Guide for Mac - Apple Support
More help / info / support / contact options etc:
Logic Pro Support Website:
Logic Pro - Official Apple Support
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All the best :-)