UWP apps have only one windows while WinUI3 can create new ones. How do I open a new Window in WinUI3 with WinRT/C++?.

Now, to move the window to another screen, in theory, you should be able to use WinUI3's DisplayArea class.

But the problem is its FindAll method is broken (for some time now...) as reported here; Microsoft.UI.Windowing.DisplayArea.FindAll iterator throws "Invalid Cast

There's also WinRT's DisplayManager, DisplayTarget, etc. but they're not super easy to use... And we can't use Winforms' Screen class...

So, one other solution is to use "classic" .NET interop, like this:

var window = new Window();
window.Content = new TextBlock() { Text = "Hello" };
window.Activate();

var monitors = Monitor.All.ToArray();
if (monitors.Length > 1)
{
    var thisMonitor = Monitor.FromWindow(WinRT.Interop.WindowNative.GetWindowHandle(this));
    var otherMonitor = monitors.First(m => m.DeviceName != thisMonitor.DeviceName);
    // move to second display's upper left corner
    window.AppWindow.Move(new PointInt32(otherMonitor.WorkingArea.X, otherMonitor.WorkingArea.Y));
}

With this Monitor class:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Runtime.InteropServices;
using Windows.Graphics;

public sealed class Monitor
{
    private Monitor(IntPtr handle)
    {
        Handle = handle;
        var mi = new MONITORINFOEX();
        mi.cbSize = Marshal.SizeOf(mi);
        if (!GetMonitorInfo(handle, ref mi))
            throw new Win32Exception(Marshal.GetLastWin32Error());

        DeviceName = mi.szDevice.ToString();
        Bounds = new RectInt32(mi.rcMonitor.left, mi.rcMonitor.top, mi.rcMonitor.right - mi.rcMonitor.left, mi.rcMonitor.bottom - mi.rcMonitor.top);
        WorkingArea = new RectInt32(mi.rcWork.left, mi.rcWork.top, mi.rcWork.right - mi.rcWork.left, mi.rcWork.bottom - mi.rcWork.top);
        IsPrimary = mi.dwFlags.HasFlag(MONITORINFOF.MONITORINFOF_PRIMARY);
    }

    public IntPtr Handle { get; }
    public bool IsPrimary { get; }
    public RectInt32 WorkingArea { get; }
    public RectInt32 Bounds { get; }
    public string DeviceName { get; }

    public static IEnumerable<Monitor> All
    {
        get
        {
            var all = new List<Monitor>();
            EnumDisplayMonitors(IntPtr.Zero, IntPtr.Zero, (m, h, rc, p) =>
            {
                all.Add(new Monitor(m));
                return true;
            }, IntPtr.Zero);
            return all;
        }
    }

    public override string ToString() => DeviceName;
    public static IntPtr GetNearestFromWindow(IntPtr hwnd) => MonitorFromWindow(hwnd, MFW.MONITOR_DEFAULTTONEAREST);
    public static IntPtr GetDesktopMonitorHandle() => GetNearestFromWindow(GetDesktopWindow());
    public static IntPtr GetShellMonitorHandle() => GetNearestFromWindow(GetShellWindow());
    public static Monitor FromWindow(IntPtr hwnd, MFW flags = MFW.MONITOR_DEFAULTTONULL)
    {
        var h = MonitorFromWindow(hwnd, flags);
        return h != IntPtr.Zero ? new Monitor(h) : null;
    }

    [Flags]
    public enum MFW
    {
        MONITOR_DEFAULTTONULL = 0x00000000,
        MONITOR_DEFAULTTOPRIMARY = 0x00000001,
        MONITOR_DEFAULTTONEAREST = 0x00000002,
    }

    [Flags]
    public enum MONITORINFOF
    {
        MONITORINFOF_NONE = 0x00000000,
        MONITORINFOF_PRIMARY = 0x00000001,
    }

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
    private struct MONITORINFOEX
    {
        public int cbSize;
        public RECT rcMonitor;
        public RECT rcWork;
        public MONITORINFOF dwFlags;

        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
        public string szDevice;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct RECT
    {
        public int left;
        public int top;
        public int right;
        public int bottom;
    }

    private delegate bool MonitorEnumProc(IntPtr monitor, IntPtr hdc, IntPtr lprcMonitor, IntPtr lParam);

    [DllImport("user32")]
    private static extern IntPtr GetDesktopWindow();

    [DllImport("user32")]
    private static extern IntPtr GetShellWindow();

    [DllImport("user32")]
    private static extern bool EnumDisplayMonitors(IntPtr hdc, IntPtr lprcClip, MonitorEnumProc lpfnEnum, IntPtr dwData);

    [DllImport("user32")]
    private static extern IntPtr MonitorFromWindow(IntPtr hwnd, MFW flags);

    [DllImport("user32", CharSet = CharSet.Unicode)]
    private static extern bool GetMonitorInfo(IntPtr hmonitor, ref MONITORINFOEX info);
}