item_Expand analogue in MVVM - wpf

Question

I am writing an application like folder explorer using MWF(MVVM pattern).
Previously I wrote it without using an MVVM, and I am curious how should I do it in MVVM approach. The problem is, that I want my application to load the "children" nodes, only when the "parent" node is clicked (expand button). Now I have this piece of code Model:
public class Node : ViewModelBase
{
public string Name
{
get => name;
set
{
name = value;
OnPropertyChanged("Name");
}
}
public long Size
{
get => size;
set
{
size = value;
OnPropertyChanged("Size");
}
}
private List<Node> nodes;
private string name;
private long size;
public List<Node> Nodes
{
get
{
return nodes;
}
set
{
nodes = value;
OnPropertyChanged("Nodes");
}
}
}
ViewModel:
public class MainWindowViewModel : ViewModelBase
{
private List<Node> nodes;
public MainWindowViewModel()
{
var logicDrives = DriveInfo.GetDrives();
Nodes = new();
foreach (var drive in logicDrives)
{
var node = new Node
{
Name = drive.Name,
Size = (long)default,
Nodes = new()
{
new Node{Name = "dummy", Size = 12, Nodes = new()}
}
};
Nodes.Add(node);
}
}
public List<Node> Nodes
{
get => nodes;
set
{
nodes = value;
OnPropertyChanged("Drives");
}
}
}
XAML:
<Window.DataContext>
<VM:MainWindowViewModel/>
</Window.DataContext>
<Grid>
<TreeView x:Name="MainTreeView"
HorizontalAlignment="Stretch"
Margin="10"
VerticalAlignment="Stretch"
ItemsSource="{Binding Nodes}">
<TreeView.ItemTemplate>
<HierarchicalDataTemplate DataType="{x:Type VM:Node}"
ItemsSource="{Binding Path=Nodes, UpdateSourceTrigger=PropertyChanged}">
<StackPanel Orientation="Horizontal">
<TextBlock Width="200"
Text="{Binding Name, UpdateSourceTrigger=PropertyChanged}"
Margin="0 0 10 0" />
<TextBlock Width="100"
Text="{Binding Size, UpdateSourceTrigger=PropertyChanged}" />
</StackPanel>
</HierarchicalDataTemplate>
</TreeView.ItemTemplate>
</TreeView>
</Grid>
In my ViewModel, I've created the drives, and populate them with "dummys" in order to make them expandable, now it looks like this
And I want to load the "child nodes" instead of dummys, when clicking on the expand button.
P.S: I already have all functions for GetDirectories, and CalculateDirectories size, I just don't understand how to call them, when some node is clicked. THX