Xamarin.Forms doesn't support closing modal page out of the box. We have to implement this feature manually.
This sample project presents how to implement a close button on a modal page in Xamarin.Forms. The close button is implemented according to the guidelines provided by Apple and Google.
Google docs about dialogs: https://material.io/design/components/dialogs.html
Apple docs about modality: https://developer.apple.com/design/human-interface-guidelines/ios/app-architecture/modality/