How can I customize an NSToolbarItem to make it look completely different?

Question

I'm programming an iOS app to run on macOS via MacCatalyst.

I know how to create an NSToolbar and populate it with all sorts of premade items using this code:

SceneDelegate.h

- (void)scene:(UIScene *)scene willConnectToSession:(UISceneSession *)session options:(UISceneConnectionOptions *)connectionOptions {
    // Use this method to optionally configure and attach the UIWindow `window` to the provided UIWindowScene `scene`.
    // If using a storyboard, the `window` property will automatically be initialized and attached to the scene.
    // This delegate does not imply the connecting scene or session are new (see `application:configurationForConnectingSceneSession` instead).
    
#if TARGET_OS_MACCATALYST
    // Code specific to Mac.
    
    self.mainToolbar = [[NSToolbar alloc] initWithIdentifier:@"mainToolbar"];
    self.mainToolbar.delegate = self;
    
    [(UIWindowScene*)scene titlebar].separatorStyle = UITitlebarSeparatorStyleNone;
    [(UIWindowScene*)scene titlebar].toolbar = self.mainToolbar;
    [(UIWindowScene*)scene titlebar].toolbarStyle = UITitlebarToolbarStyleUnified;
    [(UIWindowScene*)scene titlebar].titleVisibility = UITitlebarTitleVisibilityHidden;
    
#else
    // Code to exclude from Mac.
#endif

}

- (NSToolbarItem *)toolbar:(NSToolbar *)toolbar
     itemForItemIdentifier:(NSToolbarItemIdentifier)itemIdentifier
 willBeInsertedIntoToolbar:(BOOL)flag {
    
    NSToolbarItem *testButton = [[NSToolbarItem alloc] initWithItemIdentifier:@"button1"];
    
    testButton.title = @"Test";
    
    return testButton;
}

It looks like this:

But what I'm looking for is to be able to create my own custom NSToolbarItem that looks completely different. For example, Chrome on macOS is obviously using a custom NSToolbar or NSToolbarItem in order to create the tab bar:

According to Apple, the NSToolbarItem has a view property that I can play with? This view is nowhere to be found in Xcode: