SHANGHAI—Chinese technology giants Baidu Inc., Alibaba Group Holding Ltd. and Tencent Holdings Ltd.—dominant forces in e-commerce, mobile payments and social media—now are elbowing their way into autos, threatening established car makers.

These three companies—the so-called BATs—are plowing millions of dollars into electric-vehicle startups, car-sharing services and online retailers, as well as software platforms for autonomous driving and online car selling.

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