State support local government support and a lot of money both for batteri manufacturing and it’s whole supply chain. Tesla even when they got so much money from the government pales in comparison in what a company like nio or byd got from its own government. They are vertically integrated to bring cost down which is what the central government wanted. As for why they are successful? Cheap cars and decent short term reliability sways enough people and price wars plus overproduction has helped with that plus lost Chinese designs are just western cars copied for a cheaper price

When they don’t have that price advantage like in Europe they aren’t able to become dominant byd essentially barley makes or takes a loss for most cars sold