If you absolutely need GarageBand’s interface and loops, consider (if hardware permits) to at least macOS Mojave (10.14) or newer, where GarageBand is freely downloadable from the Mac App Store.
"The current version requires macOS 11 or later, but you can download the last compatible version." The "Workaround" for New Users garageband mac os x el capitan 10116 download extra verified