This is a sample how to leverage StoreKit2 in Xamarin.iOS. Extend to your needs.
The methods need to be objc compatibile (@objc), class has to derive from NSObject so it can be consumed from objc. I just added showManageSubscriptions because this is the only method I needed in my project.
xcodebuild -create-xcframework -framework path-from-your-xcode-derived-path/Release-iphoneos/StoreKit2.framework -framework Release-path-from-your-xcode-derived-path//StoreKit2.framework -output ./StoreKit2.xcframework
https://github.com/xamarin/xamarin-macios/blob/main/Make.config#L234C1-L235C1
sharpie bind ..../Release-iphoneos/StoreKit2.framework/Headers/StoreKit2-Swift.h --sdk iphoneos16.2
Take a look at .nuspec and .target file. This is required so .xcframework is properly included in your iOS project.
There is Xamarin iOS bug that prevents async swift methods working on simulator. It works on real device though which is enough for StoreKit. xamarin/xamarin-macios#18562