var result = await ImageSource.GetPlatformImageAsync(MauiContext);
var icon = result.Value; // UIImageを取り出す

public partial class MyTabbedPage: TabbedPage
{
    public MyTabbedPage()
    {
#if ANDROID
        this.HandlerChanged +=MyTabbedPage_HandlerChanged;
        this.HandlerChanging += MyTabbedPage_HandlerChanging;
#endif
    }
}

public partial class MyTabbedPage
{
    BottomNavigationView _bottomNavigationView;
    BottomNavigationView BottomNavigationView => _bottomNavigationView ??= GetBottomNavigationView();

    private void MyTabbedPage_HandlerChanged(object sender, EventArgs e)
    {       
        // ここでBottomNavigationViewをあれこれ頑張る
    }

    private void MyTabbedPage_HandlerChanging(object sender, HandlerChangingEventArgs e)
    {
        if (e.OldHandler == null)
        {
            return;
        } 
        // 後始末
    }

    BottomNavigationView GetBottomNavigationView()
    {
        // ReflectionでBottomNavigationViewを取得する
        var tabbedPageManagerInfo = typeof(TabbedPage).GetField("_tabbedPageManager", BindingFlags.Instance | BindingFlags.NonPublic);
        var manager = tabbedPageManagerInfo.GetValue(this);

        var bottomNaviInfo = manager.GetType().GetField("_bottomNavigationView", BindingFlags.Instance | BindingFlags.NonPublic);

        return bottomNaviInfo.GetValue(manager) as BottomNavigationView;
    }

<Grid>
    <Grid Padding="30">
         <Label Text="Text" />
    </Grid>
</Grid>

public class RoundImage:ContentView
{
    public static BindableProperty SourceProperty = BindableProperty.Create(
            nameof(Source),
            typeof(ImageSource),
            typeof(RoundImage),
            default(ImageSource),
            defaultBindingMode: BindingMode.OneWay
        );

    public ImageSource Source{
        get { return (ImageSource)GetValue(SourceProperty); }
        set { SetValue(SourceProperty, value); }
    }

    Image _image;
    Frame _frame;
    Border _border;

    public RoundImage()
    {
        _image = new Image();
        _image.Aspect = Aspect.AspectFill;
        _image.SetBinding(WidthRequestProperty, new Binding("WidthRequest", source: this));
        _image.SetBinding(HeightRequestProperty, new Binding("HeightRequest", source: this));
        _image.SetBinding(Image.SourceProperty, new Binding("Source", source: this));


        // MAUIの不具合による暫定対応。
        // iOSのBorderでClipできない問題とAndroidでFrameでLayoutが崩れる問題
#if IOS
        _frame = new Frame();
        _frame.Padding = new Thickness();
        _frame.HasShadow = false;
        _frame.BorderColor = Colors.Transparent;
        _frame.IsClippedToBounds = true;
        _frame.SetBinding(WidthRequestProperty, new Binding("WidthRequest", source: this));
        _frame.SetBinding(HeightRequestProperty, new Binding("HeightRequest", source: this));
        _frame.Content = _image;
        Content = _frame;
#elif ANDROID
        _border = new Border();
        _border.Padding = new Thickness();
        _border.Stroke = Colors.Transparent;
        _border.StrokeThickness = 0;
        _border.SetBinding(Border.WidthRequestProperty, new Binding("WidthRequest", source: this));
        _border.SetBinding(Border.HeightRequestProperty, new Binding("HeightRequest", source: this));
        _border.Content = _image;
        Content = _border;
#endif
    }

    protected override void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        base.OnPropertyChanged(propertyName);
        if(propertyName == WidthRequestProperty.PropertyName)
        {
#if IOS
            _frame.CornerRadius = (float)WidthRequest / 2f;
#elif ANDROID
            _border.StrokeShape = new RoundRectangle
            {
                CornerRadius = new CornerRadius(WidthRequest / 2d)
            };
#endif
        }
    }
}

public partial class App : PrismApplication

<prism:PrismApplication xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:prism="http://prismlibrary.com"
             x:Class="Sample.App">
</prism:PrismApplication>

        await navigationService
            .CreateBuilder()                
            .AddSegment<NaviViewModel>(config =>
            {
                config
                .UseModalNavigation(true)
                .AddParameter(KnownNavigationParameters.Animated, animated);
            })
            .AddSegment<HogeViewModel>()
            .AddParameter(ParametersBase.ParameterKey, parameters)
            .NavigateAsync();

navigationService.NavigateAsync("NaviPage?useModalNavigation=True/HogePage");

public partial class BorderGrid: Grid
{
    public static BindableProperty BorderColorProperty = BindableProperty.Create(
            nameof(BorderColor),
            typeof(Color),
            typeof(BorderGrid),
            default(Color),
            defaultBindingMode: BindingMode.OneWay
        );

    public Color BorderColor{
        get { return (Color)GetValue(BorderColorProperty); }
        set { SetValue(BorderColorProperty, value); }
    }

    public static BindableProperty BorderWidthProperty = BindableProperty.Create(
            nameof(BorderWidth),
            typeof(double),
            typeof(BorderGrid),
            default(double),
            defaultBindingMode: BindingMode.OneWay
        );

    public double BorderWidth{
        get { return (double)GetValue(BorderWidthProperty); }
        set { SetValue(BorderWidthProperty, value); }
    }

    public static BindableProperty BorderRadiusProperty = BindableProperty.Create(
            nameof(BorderRadius),
            typeof(double),
            typeof(BorderGrid),
            default(double),
            defaultBindingMode: BindingMode.OneWay
        );

    public double BorderRadius{
        get { return (double)GetValue(BorderRadiusProperty); }
        set { SetValue(BorderRadiusProperty, value); }
    }

    protected override void OnHandlerChanged()
    {
        base.OnHandlerChanged();
        UpdateBorder();
    }

    protected override void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        base.OnPropertyChanged(propertyName);
        if(propertyName == BorderColorProperty.PropertyName ||
            propertyName == BorderWidthProperty.PropertyName ||
            propertyName == BorderRadiusProperty.PropertyName)
        {
            if(Handler != null)
            {
                UpdateBorder();
            }            
        }
    }
}

public partial class BorderGrid
{
    void UpdateBorder()
    {
        var handler = Handler as IPlatformViewHandler;
        handler.PlatformView.Layer.CornerRadius = (float)BorderRadius;
        handler.PlatformView.Layer.BorderWidth = (float)BorderWidth;
        handler.PlatformView.Layer.BorderColor = BorderColor.ToCGColor();
        handler.PlatformView.ClipsToBounds = true;
    }
}

public partial class BorderGrid
{
    GradientDrawable _border;

    void UpdateBorder()
    {
        var handler = Handler as IPlatformViewHandler;
        var context = handler.PlatformView.Context;
        var view = handler.PlatformView;
        _border ??= new GradientDrawable();

        var width = (int)context.ToPixels(BorderWidth);
        var radius = (int)context.ToPixels(BorderRadius);

        _border.SetStroke(width, BorderColor.ToPlatform());
        _border.SetCornerRadius(radius);
        view.SetPadding(width, width, width, width);
        view.ClipToOutline = true;

        view.Background = _border;
    }
}