You can hide expander of Xamarin.Forms SfTreeView by setting the ExpanderWidth property to zero in Xamarin.Forms.
You can also refer the following article.
https://www.syncfusion.com/kb/11422/how-to-hide-expander-in-xamarin-forms-treeview-sftreeview
XAML
Set the ExpandActionTarget to Node to expand or collapse the TreeViewNode.
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:TreeViewXamarin"
xmlns:syncfusion="clr-namespace:Syncfusion.XForms.TreeView;assembly=Syncfusion.SfTreeView.XForms"
x:Class="TreeViewXamarin.MainPage">
<ContentPage.BindingContext>
<local:FileManagerViewModel x:Name="viewModel"/>
</ContentPage.BindingContext>
<StackLayout>
<syncfusion:SfTreeView x:Name="treeView" ExpanderWidth="0" ExpandActionTarget="Node" ChildPropertyName="SubFiles" ItemTemplateContextType="Node" ItemsSource="{Binding ImageNodeInfo}">
<syncfusion:SfTreeView.ItemTemplate>
<DataTemplate>
<Grid x:Name="grid" RowSpacing="0" >
<Grid.ColumnDefinitions>
<ColumnDefinition Width="40" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Image Source="{Binding Content.ImageIcon}" VerticalOptions="Center" HorizontalOptions="Center" HeightRequest="35" WidthRequest="35"/>
<Grid Grid.Column="1" RowSpacing="1" Padding="1,0,0,0" VerticalOptions="Center">
<Label LineBreakMode="NoWrap" Text="{Binding Content.ItemName}" VerticalTextAlignment="Center"/>
</Grid>
</Grid>
</DataTemplate>
</syncfusion:SfTreeView.ItemTemplate>
</syncfusion:SfTreeView>
</StackLayout>
</ContentPage>
Output