Complex sentence: Shanghai was particularly notable for its numerous Jewish refugees (Shanghai Ghetto), most of whom left after the war, the rest relocating prior to or immediately after the establishment of the People's Republic.
Split into simpler sentences: Shanghai was particularly notable for its numerous Jewish refugees, who gathere in the so-called Shanghai Ghetto. Most of them left China after the war, the rest relocating prior to, or immediately after, the establishment of the People's Republic.