how to set orientation of one page in maui?

You can use Invoke platform code to achieve this.Platform code can be invoked from cross-platform code by using conditional compilation, or by using partial classes and partial methods.

Here I try to use partial classes and partial methods to achieve this(Based on the official sample, I added the test code ,which could help you understand this concept.).

Please refer to the following code:

    public enum DeviceOrientation
    {
        Undefined,
        Landscape,
        Portrait
    }

namespace InvokePlatformCodeDemos.Services.PartialMethods
{
    public partial class DeviceOrientationService
    {
        public partial DeviceOrientation GetOrientation();
        public partial void SetDeviceOrientation(DisplayOrientation deviceOrientation);
    }
}

In Android

public partial class DeviceOrientationService
{
    public partial DeviceOrientation GetOrientation()
    {
        IWindowManager windowManager = Android.App.Application.Context.GetSystemService(Context.WindowService).JavaCast<IWindowManager>();
        SurfaceOrientation orientation = windowManager.DefaultDisplay.Rotation;
        bool isLandscape = orientation == SurfaceOrientation.Rotation90 || orientation == SurfaceOrientation.Rotation270;
        return isLandscape ? DeviceOrientation.Landscape : DeviceOrientation.Portrait;
    }

    private static readonly IReadOnlyDictionary<DisplayOrientation, ScreenOrientation> _androidDisplayOrientationMap =
        new Dictionary<DisplayOrientation, ScreenOrientation>
        {
            [DisplayOrientation.Landscape] = ScreenOrientation.Landscape,
            [DisplayOrientation.Portrait] = ScreenOrientation.Portrait,
        };
// method SetDeviceOrientation
    public partial void SetDeviceOrientation(DisplayOrientation displayOrientation)
    {
        var currentActivity = ActivityStateManager.Default.GetCurrentActivity();
        if (currentActivity is not null)
        {
            if (_androidDisplayOrientationMap.TryGetValue(displayOrientation, out ScreenOrientation screenOrientation))
            {
                currentActivity.RequestedOrientation = screenOrientation;
            }
        }
    }
}

In iOS

public partial class DeviceOrientationService
{
    public partial DeviceOrientation GetOrientation()
    {
        UIInterfaceOrientation orientation = UIApplication.SharedApplication.StatusBarOrientation;
        bool isPortrait = orientation == UIInterfaceOrientation.Portrait || orientation == UIInterfaceOrientation.PortraitUpsideDown;
        return isPortrait ? DeviceOrientation.Portrait : DeviceOrientation.Landscape;
    }


    private static readonly IReadOnlyDictionary<DisplayOrientation, UIInterfaceOrientation> _iosDisplayOrientationMap =
       new Dictionary<DisplayOrientation, UIInterfaceOrientation>
       {
           [DisplayOrientation.Landscape] = UIInterfaceOrientation.LandscapeLeft,
           [DisplayOrientation.Portrait] = UIInterfaceOrientation.Portrait,
       };
     // implement method SetDeviceOrientation
    public partial void SetDeviceOrientation(DisplayOrientation displayOrientation)
    {
        if (UIDevice.CurrentDevice.CheckSystemVersion(16, 0))
        {
            var scene = (UIApplication.SharedApplication.ConnectedScenes.ToArray()[0] as UIWindowScene);
            if (scene != null)
            {
                var uiAppplication = UIApplication.SharedApplication;
                var test = UIApplication.SharedApplication.KeyWindow?.RootViewController;
                if (test != null)
                {
                    test.SetNeedsUpdateOfSupportedInterfaceOrientations();
                    scene.RequestGeometryUpdate(
                        new UIWindowSceneGeometryPreferencesIOS(UIInterfaceOrientationMask.Portrait), error => { });
                }
            }
        }
        else
        {
            UIDevice.CurrentDevice.SetValueForKey(new NSNumber((int)UIInterfaceOrientation.Portrait), new NSString("orientation"));
        }
    }
}

And use it like this:

namespace InvokePlatformCodeDemos;
using PartialMethodsDeviceOrientationService = InvokePlatformCodeDemos.Services.PartialMethods.DeviceOrientationService;
public partial class NewPage : ContentPage
{
	public NewPage()
	{
		InitializeComponent();
	}
    protected override void OnAppearing()
    {
        base.OnAppearing();
        // set the Device Orientation here
        var deviceOrientationService2 = new PartialMethodsDeviceOrientationService();
        deviceOrientationService2.SetDeviceOrientation(DisplayOrientation.Landscape);
    }
}

For more information, you can refer to : NET MAUI – Locking Orientation into Portrait or Landscape and Invoke platform code.