Four years ago, Xiaomi was a successful smartphone business and its car business consisted of nothing more than a plan approved by the board and a vow by billionaire founder Lei Jun to make it work. This year, the company’s assembly lines are set to turn out 300,000 vehicles, and it has already shipped more than 135,000 in less than a year on the market. The wait list for its first car, the SU7, a Porsche look-alike that starts at around $30,000, is around half a year, and Xiaomi’s Hong Kong-listed shares have more than tripled in a year. Lei...