For example the city of Beijing has the same GDP of Sweden and Belgium and it's not far from the city of London.

So? The country itself can still be under-developed.

the median salary in US is $ 36k/year[1]

in China is $ 13.5k, but cost of living in China is more than 50% cheaper than in US on average and education is basically free

Kentucky has the same population of Switzerland but less than half average income (27k vs 62k)

14% of the population in USA relys on food stamps

OP said I would not say it is developed, just because the top percentile of the population seem to live lavish lives, does that apply to US as well? (or Italy, where I come from and similar patterns are observable)

there is more than the average and the median when 1.2 billion people live in the same country

development is not homogeneous anywhere

[1] The Bureau of Labor Statistics reported a median personal income of $865 weekly for all full-time workers in 2017. The U.S. Census Bureau lists the annual real median personal income at $35,977 in 2019 with a base year of 2019.

Well, large parts of the USA do feel like a developing countries. Some even have comparable life expectancies...

that's per household, not per capita.