Complex sentence: It was a pilot project for the development of the next four death factories; with faster methods of killing and incinerating people; the remaining three being Belzec, Sobibor, and Majdanek.
Split into simpler sentences: It was a pilot project for the development of the next four death factories; with faster methods of killing and incinerating people. The remaining three being Belzec, Sobibor, and Majdanek.