Shanghai is modern. Huge parts of the country are developing.
Whether a country is developed or not is defined based on their standard of living per capita, not their absolute size. A country with a billion poor people has completely different social, demographic, industrial and political drivers than a country of 300m high I come people.
The last thing on my mind when I think of China is "its a developing nation." China seems to own quite an empire.
Coastal China has per capita GDP comparable to Taiwan. Interior provinces is much poorer, with 600M living on 1000RMB/month or 2000USD/year, on the level of Haiti. In aggregate China is a middle income country with per capita GDP of Iraq.
The Chinese system harnesses this income disparity + 1.4 Billion people to concentrate talent, wealth and development systematically rival developed nations. That's just a factor of scale. Quantity has a quality of its own if properly coordinated, which is basically broad TLDR of Chinese history up to industrialization. I suppose people didn't think China could catch up this fast.