Musk, who first rose to prominence in Silicon Valley by co-founding PayPal Holdings Inc., sees payments as crucial to creating a so-called super app similar to social products that have flourished in China. WeChat, for example, lets users hail a ride, book a flight and pay off their credit card. As Musk told employees in February, “We want it to be such that, if you want to, you could live your life on the X app.”