
If I'm understanding my history correctly, this arc is probably set in a city like Shanghai in the 1920s-1930s. Shanghai then was glamorous and cosmopolitan, with a lot of music, art, commerce, cinema, and more going on. It would also explain why we're seeing a mix of the past and the modern, plus the East and the West, including the cars and microphones and cheongsams and Western suits.
Since it is fiction, some artistic licence is probably being taken. (Like the plane crash you mentioned. That would more likely have been a ship or train accident since plane travel wasn't common before the '50s, I think.) But I'm guessing that Old Shanghai from the 1920s is what the author is drawing inspiration from.

This is based on the Chinese Republic era which took place around 1910s to 1950.
This is when China started to industrialize itself and things like divorce and "freedom of love" become more "common" and all.
This is when the qipao starting popping up and people abandoned the old Manchu styled clothing from the Qing Dynasty.
People started to ride cars, airplanes, and mingle with the foreigners coming from England, Japan, and America among others.
You'll see both traces of the Qing dynasty and the Republic as usually settings like this focus on the change from old to new as well its impacts.
Did the author change their mind about the setting of this arc in the middle of it? It seemed like it was in the past and now it's modern. Wtf?