Xcode 11 drops support for Application Loader which was the de facto standard for uploading apps whether they were developed natively or through a cross platform framework like Xamarin. Microsoft has since updated their documentation to show how to upload apps directly via VS for Mac: https://docs.microsoft.com/en-us/xamarin/ios/deploy-test/app-distribution/app-store-distribution/publishing-to-the-app-store?tabs=macos
While this method seems convenient, it involves some additional setup. Since VS for Mac is a 3rd party application that needs to access information related to your Apple ID, you’ll need to generate an app specific password for authentication. This sounds easy, but you should note this also requires 2FA enabled on that account. I am a strong proponent for 2FA, but this may not work out so easily for everyone such as when your development device uses a different Apple ID than your Apple Developer associated Apple ID. Fortunately, there is a workaround using Xcode’s app distribution feature If you are logged into your development Apple ID in Xcode (Xcode > Preferences > Accounts), you can actually upload directly through the Organizer (Window > Organizer). As long as you have already generated the archive for publishing under VS for Mac, your archive will show up under the Xcode Organizer and you can sign and distribute the app via the usual channels (App Store, Ad Hoc, Enterprise).