How to get vtkDataSet (cells, points, ...) information from a source in VTK-9?

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Ah, I see the distinction - thanks @dgobbi .
My goal is to understand how data flows between VTK pipeline elements. Now thanks to you I see how I can inspect my TopoGridReader::GetOutput() for vtkPolyData.

A vtkElevationFilter object is next in the pipeline:

elevFilter->SetInputConnection(gridReader_->GetOutputPort());
elevFilter_->SetScalarRange(low, high); 
// Feed elevFilter output to surfaceMapper   
surfaceMapper->SetInputConnection(elevFilter->GetOutputPort());

How can I inspect the output of the vtkElevationFilter? I tried:

polyData = pipeline->elevFilter_->GetPolyDataOutput();
polys = polyData->GetPolys();
if (polys) {
  qDebug() << "#cells in elevFilter: " << polys->GetNumberOfCells();
}
else {
  qDebug() << "no polys in elevFilter output";
}
    
points = polyData->GetPoints();
if (points) {
  qDebug() << "#points in elevFilter: " << points->GetNumberOfPoints();
}
else {
  qDebug() << "no points in elevFilter output";      
}

But the debug output is:

#cells in elevFilter output:  0
no points in elevFilter output

But my VTK app displays the elevation data as I expect, so obviously data is flowing through the pipeline. What am I missing?