Sounds like an attempt at phrasing for the general public.

Android apps have zero or more activities, each of which may be thought of as a single screen and a single Intent, which is a bit like a URL (and sometimes very much like a URL). A messenger or email app will typically have a main activity, an activity to view a single message, an activity to view a conversation with someone, perhaps an activity to view a single attached image, probably an activity to view and edit the application's settings, and so on.

What is sent is perhaps the app's name and a class name within the app for each activity that's started.

Exactly right. And you don't have to be a system app to access this information. Any app with sufficient permissions granted explicitly by a user can access these data (no root needed), and it may have legitimate reasons for doing it.

It sounds a lot like the screen events Firebase reports (a library by Google for analytics, among other things)

It allows you to know which screens a user views, but not the data on the screen. A pseudo-example would be like "User opened LoginScreen/LoginActivity at yyyy-mm-dd and stayed on that screen for X seconds"

Not an actual screenshot of said screen