A prompt will appear stating the current version is incompatible but will offer to download the for your OS.
Searching for “GarageBand El Capitan 10.11.6 download extra” will lead to sketchy sites like: garageband mac os x el capitan 10116 download extra verified
If GarageBand isn't in your purchase history, you can "link" it to your account using a friend's newer Mac. A prompt will appear stating the current version
