public bool AddItem( 
   IProjectItem item
)

{
  // Add a folder connection to a project

  Item folderToAdd = ItemFactory.Instance.Create(@"C:\Data\Oregon\Counties\Streets");
  // Note: Needs QueuedTask to run
  bool wasAdded = Project.Current.AddItem(folderToAdd as IProjectItem);
  // use wasAdded;
}

{
  // Add a file geodatabase or a SQLite or enterprise database connection to a project
  Item gdbToAdd = folderToAdd.GetItems().FirstOrDefault(folderItem => folderItem.Name.Equals("CountyData.gdb"));
  // Note: Needs QueuedTask to run
  var addedGeodatabase = Project.Current.AddItem(gdbToAdd as IProjectItem);
  // use addedGeodatabase;
}

{
  // Import a mxd
  Item mxdToImport = ItemFactory.Instance.Create(@"C:\Projects\RegionalSurvey\LatestResults.mxd");
  // Note: Needs QueuedTask to run
  var addedMxd = Project.Current.AddItem(mxdToImport as IProjectItem);

  // Add map package
  Item mapPackageToAdd = ItemFactory.Instance.Create(@"c:\Data\Map.mpkx");
  // Note: Needs QueuedTask to run
  var addedMapPackage = Project.Current.AddItem(mapPackageToAdd as IProjectItem);

  // Add an exported Pro map
  Item proMapToAdd = ItemFactory.Instance.Create(@"C:\ExportedMaps\Election\Districts.mapx");
  // Note: Needs QueuedTask to run
  var addedMapProjectItem = Project.Current.AddItem(proMapToAdd as IProjectItem);
}

{
  //Create a new GDB item and add it to the project
  if (ItemFactory.Instance.Create(newGDDItemPath) is not IProjectItem newGDBItem)
  {
    // could not create the item
    return;
  }
  var success = Project.Current.AddItem(newGDBItem);
  //make the newly added GDB item the default
  if (success)
    Project.Current.SetDefaultGeoDatabasePath(newGDDItemPath);
  //Now remove the old item
  if (Project.Current.GetItems<Item>().FirstOrDefault(i => i.Path == oldGDBItemPath) is not IProjectItem oldGDBItem)
  {
    // could not find the item
    return;
  }
  var removeSuccess = Project.Current.RemoveItem(oldGDBItem);
}

{
  //Adding a folder connection

  //Create the folder connection project item
  var item = ItemFactory.Instance.Create(folderPath) as IProjectItem;

  // Note: Needs QueuedTask to run
  var folder = Project.Current.AddItem(item) ? item as FolderConnectionProjectItem : null;
}

{
  //Adding a Geodatabase:
  //Create the File GDB project item
  // Note: Needs QueuedTask to run
  var item = ItemFactory.Instance.Create(gdbPath) as IProjectItem;
  var newlyAddedGDB = Project.Current.AddItem(item) ? item as GDBProjectItem : null;
}

{
  var templateFileName = "Layout.pagx";
  // Get layout Template Path from the project's home folder and combine it with a file name
  var projectPath = CoreModule.CurrentProject.HomeFolderPath;
  var layoutTemplateFilePath = System.IO.Path.Combine(projectPath, templateFileName);
  // Create a new layout project item with the layout file path
  // Create an IProjectItem using a layout template pagx file
  IProjectItem pagx = ItemFactory.Instance.Create(layoutTemplateFilePath) as IProjectItem;
  // Add the IProjectItem to the current project
  //Note: Needs QueuedTask to run
  Project.Current.AddItem(pagx);
}

{
  //Create a layout project item from importing a pagx file
  //Note: Needs QueuedTask to run
  IProjectItem pagx = ItemFactory.Instance.Create(
                            @"C:\Temp\Layout.pagx") as IProjectItem;
  Project.Current.AddItem(pagx);
}