Complex sentence: The Institute of National Remembrance reported that the operations of "Intelligenzaktion" killed 100,000 Polish people; by way of mass disappearances, the Nazis collected, transported to, and killed the selected peoples in remote places, and buried them in mass graves.
Split into simpler sentences: The Institute of National Remembrance reported that the operations of "Intelligenzaktion" killed 100,000 Polish people. By way of mass disappearances, the Nazis collected, transported to, and killed the selected peoples in remote places, and buried them in mass graves.