Apple and Google, two historic rivals, came together for the sake of public health. Many were surprised by the turn of events. Here's how it happened.

Within a few weeks, the Apple project – code-named “Bubble” – had dozens of employees working on it with executive-level support from two sponsors: Craig Federighi, a senior vice president of software engineering, and Jeff Williams, the company’s chief operating officer and de-facto head of healthcare. By the end of the month, Google had officially come on board, and about a week later, the companies’ two CEOs Tim Cook and Sundar Pichai met virtually to give their final vote of approval to the project.