Xamarin Forms iOS RootViewController.View.AddSubview() not showing element in a page with Shell.PresentationMode="ModalAnimated"

Question

I'm trying to add a floating Button for all my screens in a Xamarin.forms app using Dependency service for each platform. iOS is working fine with any ContentPage but it fails when I set Shell.PresentationMode="ModalAnimated" , button is not showing or is printing behind of ContentPage with animated modal.

Calling dependency service in my all views in Xamarin Forms

DependencyService.Get<IDeviceService>().ShowFloatingButton();

Dependency service implementation for iOS

namespace WebviewSample.iOS.DependencySeivices
{
    public class DeviceService : IDeviceService
    {
        private void ShowFloatingButton()
        {
            var vc = UIApplication.SharedApplication.KeyWindow.RootViewController;
            var btn = UIButton.FromType(UIButtonType.System);
            btn.Frame = new CGRect(20, 200, 280, 44);
            btn.SetTitle("Click Me", UIControlState.Normal);
            vc.View.BackgroundColor = UIColor.DarkGray;
            vc.View.AddSubview(btn);
            vc.View.BringSubviewToFront(btn);
        }
    }
}

I have tried to get the top controller as below samples:

    if (UIApplication.SharedApplication.KeyWindow.WindowLevel != UIWindowLevel.Normal)
    {
        foreach (var item in UIApplication.SharedApplication.Windows)
        {
            if (item.WindowLevel == UIWindowLevel.Normal)
            {
                vc = item.RootViewController;
                break;
            }
        }
    }

Also

    while (vc.PresentedViewController != null) 
    {
        vc = vc.PresentedViewController;
    }

Answer 1

According to this document: https://devblogs.microsoft.com/xamarin/xamarin.forms-shell-quick-tip-modal-navigation/

Xamarin shell navigation is different than traditional navigation where a page is pushed onto the stack so a new page pops up over the current page.