Yesterday, I wrote an absolutely amazing (if I must say so) story on how Google ranks app content at Search Engine Land. You have to read it but let me give you the short version here.
Google uses the Google App Indexing API (which I covered here a billion times) to understand the content and user metrics within the app. In the app indexing API, developers send Google the URI, title and option description of the content for that URI within the app (think of it as an XML Sitemap feed, if you will). That feeds Google the content within the app, but Google does have the ability to crawl Android apps as well.
In addition, now this is the big one, Google also has developers send the start and end time stamps of each action (fragment, segment, click or whatever you want to call it) of the users within the app. So Google knows how much time and which URIs users are going to and spending on within a specific app.
That is clear user metric data, which Google has said time and time again, that they do not use for ranking web related content. But because Google doesn't have PageRank, links, anchors as a ranking signal for ranking app content, Google needed to find something to replace it with for their app ranking and that is where the Google App Indexing API comes in.
Of course, folks like Glenn Gabe picked up on it and said, you believe to tell me that Google doesn't use this for web ranking as well? Here are the sets of tweets:
Have fun with this!
Forum discussion at Twitter.