usdz to stl conversion is used for the purpose of export of 3D models from the Universal Scene Description 3D format to STL format, typically for 3D printing.
Whilst in the past the support for .usdz files was low, nowadays it is supported in pretty much every mainstream 3D modeling tool, especially on macOS. In any case, Blender is most likely your best choice how to do this, it's free and you can find plenty of tutorials on the Internet.
Updated: September 19, 2023