Did all the people in Shanghai immigrate from Jiangbei before?

Not exactly. A few years before and after liberation, a large number of Jiangbei people fled to Shanghai, and most of them lived in Zhabei area (shanty towns). At that time, Zhabei was not a real urban area. Until now, most Jiangbei people still live in Zhabei. There are not many real Shanghainese, accounting for less than one-third of Shanghainese now. Before and at the beginning of liberation, a large number of Ningbo people and Shaoxing people poured into Shanghai. Now most people in Shanghai are descendants of these people. As for the suburbs, it must be Shanghai dialect, but the Shanghai dialect in the suburbs is different from that in the urban area.