Hi Confidential_Persona,
Thanks for posting in Apple Support Communities.
We understand that you're unable to download Garageband from the App Store. If you previously downloaded the app with a specific app and you want to download it again, you will need to know the account you previously used.
If you forget your Apple ID or password
Your Apple ID is your user name for the App Store, iTunes Store, Apple Books, iCloud, and other Apple services. It’s also used to manage your Apple ID settings on your Mac.
If you forget your Apple ID or password, you can reset them using Apple ID settings on your Mac.
1. On your Mac, choose Apple menu
> System Settings, then click Sign in with your Apple ID at the top of the sidebar.
2. Click “Forgot Apple ID or password?,” then do one of the following:
* If you forgot your Apple ID: Click Forgot your Apple ID?, then follow the onscreen instructions.
* If you know your Apple ID: Enter your Apple ID, click Continue, then follow the onscreen instructions.
You can find these steps in If you forget your Apple ID or password - Apple Support.
If you have the correct Apple ID and password, but unable to download due to your iOS version being too old for the available version, there would be no way to download the version available on the App Store until you have a device with a compatible version.
Regards.