I wonder where in the Apple Documentation this solution is documented. It seems kind of hard to find there.

https://developer.apple.com/documentation/swiftui/localizedstringkey/stringinterpolation/appendinterpolation(_:specifier:)/ https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFStrings/formatSpecifiers.html#//apple_ref/doc/uid/TP40004265