I have a early 2013 macbook air. Will my old GarageBand files run on a new macbook air 2020 using GarageBand?

If you are already using GarageBand 10 on macOS 11.6, your current projects should be compatible with macOS 12 on a new Mac. Just transfer your complete loop library and the instrument library to your new Mac. Don't make a clean install when you set up your new Mac but transfer everything from the old Mac to the new Mac using the Setup Assistant. Then all loops and sounds you collected over the years will remain available in the new GarageBand version.