boblishman said "Sign in with YOUR Apple ID, then go to your “Purchased” Section. Garage band should appear there with the option to re-download."
That's what worked for me with a 2017 MacBook Pro running Catalina 10.15.7
I went to the purchase section and there was an icon for Garageband and a 'cloud' icon to download it. If I clicked on the Garageband icon it took me to a page to download the Big Sur version. If I click on the 'cloud' download, a popup said, there's a newer version or continue to load the most current version for my system. I took the 'my system' path and Garageband loaded and works. I can't tell you what version it is because the 'about Garageband' is greyed out hence not selectable.
But if I do a get info on the .app file it shows version 10.3.5, created Aug. 22, 2019