Why is Elon Musk suddenly thinking about creating an “everything app,” and what does that even mean? The question arose after the billionaire chief executive of Tesla Inc earlier this month reversed course on his decision not to buy Twitter Inc. Musk completed his $44-billion acquisition of Twitter on Thursday, but provided little clarity on how he will achieve the ambitions he has outlined for the influential social media platform. The concept of an everything app, often referred to as a “super app,” is massively popular in Asia and tech companies across the world have tried to replicate it. What...