Las,
” source of Rav Moshe’s view as there is no park or market with the accumulation of 600k people every day in Brooklyn and no street that contains that”
Rav Moshe upheld that 600K does not necessarily have to be applied as the above condition, it is enough that the city in itself contains 600k, or that there are 600K within 12 MIL x 12 Mil.
However, many disagreed, and that the Shu”A simple understanding is that Rh”R of 600k is conditional on 600k passing by one place/street etc. (So was the understanding of the Mishkenos Yaakov regarding the Shu”A and Tur.)
See more on the below thread.