Finding and Selecting all instances of an element

[Sciender]

     Hi, so I'm new to the Revit API and Revit in general, so sorry if this is in the wrong place or formatted weird.

So, I'm trying to make an add-in that will take a group, find all instances of a specific element, then duplicate the original group on top of the center of the found elements.  In this case, I want to find all the Ceiling-Hosted Air Terminals in my drawing and duplicate the group on top of their center point.  I'm using the My First Revit Plugin lessons as sort of a base, but it's not much help for what I'm trying to do.  The code I have already is copy/pasted over from these lessons, but I'm not entirely sure how it ties together, so I can't use it very effectively.  If the answer is too much to put here, I understand, but it would be a huge help just to be pointed in the right direction to research the solution myself, as that hasn't given me much either.  I'll attach my existing code in a .txt file, but I'm making the program in Visual Studio 2022.  Again, sorry if this is formatted weird or in the wrong spot.

[SimonaQQ]

Hi,

According to my understanding:

    [Transaction(TransactionMode.Manual)]
    public class CommandTest : IExternalCommand
    {
        public Result Execute(ExternalCommandData commandData, ref string message, ElementSet elements)
        {
            var uidoc = commandData.Application.ActiveUIDocument;
            var doc = uidoc.Document;
            var selection = uidoc.Selection;

            //select group
            var group = doc.GetElement(selection.PickObject(ObjectType.Element, new GroupSelectionFilter())) as Group;            
            var groupLocation = (group.Location as LocationPoint).Point;

            //find all the Air Terminals in my drawing,Here I take duct as an example
            var ducts = new FilteredElementCollector(doc, doc.ActiveView.Id).OfClass(typeof(Duct)).Cast<Duct>().ToList();

            if (ducts != null && ducts.Count > 0)
            {
                //Copy the group to the desired location,Here I take the end of duct as an example
                using (Transaction tra = new Transaction(doc, "CopyGroups"))
                {
                    tra.Start();

                    ducts.ForEach(t =>
                    {
                        var ductLocationLine = (t.Location as LocationCurve).Curve as Line;

                        var end0 = ductLocationLine.GetEndPoint(0);
                        //the end of duct
                        var end1 = ductLocationLine.GetEndPoint(1);
                        //copy
                        var newGroup = ElementTransformUtils.CopyElement(doc, group.Id, end1.Subtract(groupLocation));
                    });

                    tra.Commit();
                }
            }

            return Result.Succeeded;
        }
    }

    public class GroupSelectionFilter : ISelectionFilter
    {
        public bool AllowElement(Element elem)
        {
            if (elem is Group)
            {
                return true;
            }
            return false;
        }

        public bool AllowReference(Reference reference, XYZ position)
        {
            return true;
        }
    }

[Sciender]

Thank you for the help.

Unfortunately, the point of this plugin is to place hanger groups on ceiling-hosted Air Terminals, not just air terminals in general.  What would help me a lot is just a way to find all of the ceiling-hosted air terminals and their center point, and I can probably go from there.  While it wasn't what I needed, I greatly appreciate the time you took to type out that solution.  I'll go ahead and edit my original post to better represent my issue.