If you have previously "purchased" or downloaded GarageBand on your Apple ID in the past, this is the easiest method.
If you’ve never "purchased" GarageBand (even though it's free), you can "buy" it on a friend’s newer Mac using your Apple ID. Once it’s linked to your account, it will show up in your Catalina App Store's purchased list, allowing you to trigger the legacy download. Why Catalina 10.15.7 is a "Sweet Spot"
Drainage Derbyshire