EXCLUSIVE:

A former exec for Phunware, which made Trump's app, says the company's political pitch was that it could replace the "Graph API" that allowed Cambridge Analytica to collect a wealth of information about millions of people and their connections https://www.cbsnews.com/news/trump-campaign-app-data-americans-gold-mine-phunware/

So let's go through this a bit. As of 6/30, Trump's app was on as many as 1.4 million phones. Biden's was on just 64,000, according to @Apptopia data provided to CBS News.
They both ask for access to your phone's contact lists, but Trump's app is taking in an incomparable amount.

That's a big deal. They can essentially map out a person's real life social networks, which tells a great deal about who's persuadable and why, especially when paired with other data.

The Trump app takes in a ton of other data through permissions:

But the company that developed the Trump app also says it has a massive stockpile of data to work with. Phunware says it has the ability to collect data from more than one billion active mobile devices per month.

Former Phunware exec Ian Karnell led an internal investigation that became the focal point of a lawsuit alleging fraud at the company filed by Uber. He's referred to as "Employee B" in the suit, but agreed to be identified by CBS News. Here's how he describes the political pitch:

Karnell said one of Phunware's major offerings is data drawn from proximity beacons, devices that use Bluetooth to collect information about phones that pass nearby. Useful if you want to know who attended a rally in '16 but not '20.

Here's how their value was demonstrated:

Finally, here's a graph provided by @Apptopia. It details the significant continuing edge Trump has in terms of downloads.