Complex sentence: Supposedly, they were in Belsen to recover and then to return to their original camps, and to resume work; but many of them died in Belsen of disease, starvation, exhaustion and lack of medical attention.
Split into simpler sentences: Supposedly, they were in Belsen to recover and then to return to their original camps, and to resume work. However, a large number of them actually died of disease, starvation, exhaustion and lack of medical attention.