As already metioned here I can not build this repo with Swift 5.9.1. I get the following error when I type swift build --product UICatalog.

error: 'swift-win32': package 'package@swift-5.9.swift' is using Swift tools version 3.1.0 which is no longer supported; consider using '// swift-tools-version: 5.9' to specify the current tools version

I was only able to fix it by copy Package@swift-5.4.swift to Package@swift-5.9.swift and change the swift-tools-version to 5.9.

Package.swift should be chosen by swift and the swift-tools-version 5.7.0 there should be fine when working correctly as @compnerd mentioned.