Amazon.com Inc. AMZN, -6.72% disclosed overnight that it boosted its stake in electric-vehicle maker Rivian Automotive Inc. RIVN, -4.12% to more than 18% and initiated a stake in self-driving software company Aurora Innovation Inc. AUR, -2.64% of more than 5%. Amazon said late Wednesday it now owns 162,086,884 shares of Rivian, or 18.1% of the shares outstanding, which at current prices would be valued at about $9.75 billion at current stock prices.