There is no ELI5 answer for something with so many political, social and economic dimensions. But here are possibly some points to start with.
1) Political structure. China has a far more centralized authority structure. This is somewhat “efficient” in that decisions made can be more quickly implemented without needing to obtain much popular support. India is a democracy with some power devolved to local states/provinces. This gets really messy with many local power structures opposing central government policies.
2) Economic policy. Until the 1970’s China and India were broadly aligned (ELI5 so this is an oversimplification) in terms of economic policy. Self sufficiency, import substitution and state control. Imports and exports were restricted. Foreign capital (and therefore technology transfer) is slowed down by a lot. By the late 1980’s China had started to embark on a far more outward looking stance. Allowing and encouraging foreign investments. India did not really open up until much later. India has way too many subsistence like, small inefficient farms etc and a system that is rather unproductive.
3) Social structure. The communist govt in China pretty much destroyed the old imperialist system and repressed religion and killed off the landlord class post 1949. The Mao government supported lots of education, more or less egalitarian policies, and some technocratic/meritocratic administration (at least in the Maoist framework). India’s social system resulted in very poor female workforce participation, low education levels on average and the caste society (to be very blunt) made meritocracy a bit of a joke. Another advantage, although it shouldn’t be overstated, that China had is that 90% of the people consider themselves Han chinese.
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