What internal mechanism triggers a ListBoxItem to be selected?

I'm trying to understand the details of WPF,but hit a stumbling block. I know that Selector is an abstract class that inherits from ItemsControl and adds the ability to select from its children. I know that ItemsControls have ItemTemplates that can set a child item's appearance and a panel that arranges those items. I know that there is often an ItemContainer that (at least in the case of a ListBoxItem) has an IsSelected property, which I believe can trigger the ListBox selection changed event (that might not be the right name).

What I absolutely can't figure out is, what changes the state of IsSelected internally? You click on an item, so there is probably a mouse down or preview mouse down or something...then a hit test?

I've been combing through ListBox, TabControl, and Selector source code at https://referencesource.microsoft.com/ trying to find the source of the click...is it on the panel? Is there some outside source? Do I have a fundamental misunderstanding of what's going on here?

ListBoxItem has overridden OnMouseLeftButtonDown and OnMouseRightButtonDown methods that call a private HandleMouseButtonDown method, which in turn calls an internal NotifyListItemClicked method of the ListBox class, which finally sets the item's IsSelected property.

Copied from the reference source:

internal void NotifyListItemClicked(ListBoxItem item, MouseButton mouseButton)
{
    // When a ListBoxItem is left clicked, we should take capture
    // so we can auto scroll through the list.
    if (mouseButton == MouseButton.Left && Mouse.Captured != this)
    {
        Mouse.Capture(this, CaptureMode.SubTree);
        SetInitialMousePosition(); // Start tracking mouse movement
    }

    switch (SelectionMode)
    {
        case SelectionMode.Single:
            {
                if (!item.IsSelected)
                {
                    item.SetCurrentValueInternal(IsSelectedProperty, BooleanBoxes.TrueBox);
                }
                else if ((Keyboard.Modifiers & ModifierKeys.Control) == ModifierKeys.Control)
                {
                    item.SetCurrentValueInternal(IsSelectedProperty, BooleanBoxes.FalseBox);
                }

                UpdateAnchorAndActionItem(ItemInfoFromContainer(item));
            }
            break;

        case SelectionMode.Multiple:
            MakeToggleSelection(item);
            break;

        case SelectionMode.Extended:
            // Extended selection works only with Left mouse button
            if (mouseButton == MouseButton.Left)
            {
                if ((Keyboard.Modifiers & (ModifierKeys.Control | ModifierKeys.Shift)) == (ModifierKeys.Control | ModifierKeys.Shift))
                {
                    MakeAnchorSelection(item, false);
                }
                else if ((Keyboard.Modifiers & ModifierKeys.Control) == ModifierKeys.Control)
                {
                    MakeToggleSelection(item);
                }
                else if ((Keyboard.Modifiers & ModifierKeys.Shift) == ModifierKeys.Shift)
                {
                    MakeAnchorSelection(item, true);
                }
                else
                {
                    MakeSingleSelection(item);
                }
            }
            else if (mouseButton == MouseButton.Right) // Right mouse button
            {
                // Shift or Control combination should not trigger any action
                // If only Right mouse button is pressed we should move the anchor
                // and select the item only if element under the mouse is not selected
                if ((Keyboard.Modifiers & (ModifierKeys.Control | ModifierKeys.Shift)) == 0)
                {
                    if (item.IsSelected)
                        UpdateAnchorAndActionItem(ItemInfoFromContainer(item));
                    else
                        MakeSingleSelection(item);
                }
            }

            break;
    }
}