MainWindow.xaml

<?xml version="1.0" encoding="utf-8"?>
<Window
    x:Class="winUI3.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:winUI3"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">
    <NavigationView>
        <NavigationView.MenuItems>
            <NavigationViewItem Content="Home" Icon="Home" ToolTipService.ToolTip="Home"/>
            <NavigationViewItem Content="Collections" Icon="Keyboard" ToolTipService.ToolTip="Collections">
                <NavigationViewItem.MenuItems>
                    <NavigationViewItem Content="TEST1" Icon="Page" ToolTipService.ToolTip="Page"/>
                    <NavigationViewItem Content="TEST2" Icon="Mail" ToolTipService.ToolTip="Mail"/>
                </NavigationViewItem.MenuItems>
            </NavigationViewItem>
        </NavigationView.MenuItems>
    </NavigationView>
</Window>