Historically, the app downloaded this automatically upon first launch. Because Apple has updated its content delivery networks, the in-app downloader may fail on El Capitan. How to force download the extra package:
If you have never downloaded GarageBand on your personal Apple ID, you cannot use the standard purchase history trick. However, Apple’s Family Sharing feature provides a legitimate workaround. garageband mac os x el capitan 10116 download extra verified
Unofficial versions cannot connect securely to Apple's servers to download free loops, instruments, or stability patches. This link or copies made by others cannot be deleted
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