It's built into the iOS app.
@MarkJFine - IIRC, the iOS data usage app only tracks 2 things - cellular and 'wi-fi assist' and Apple has (to my knowledge) walled off 3rd party apps from allowing you to track your wi-fi usage. This is just a bit of "last-I-heard".
I may have misunderstood the question to be a comparison of capabilities between the iOS and desktop applications.
Perhaps I should have responded that the current iOS app does everything the desktop Usage Meter application does wrt to displaying usage donuts, plan status, data token purchase/availability, plan history graph, and video settings; as well as providing a portable wifi strength gauge (really helpful for finding dead spots in the house, imo) and the ability to review billing info.
Edit: But as you say, the internal iOS ability to measure data usage wrt wireless vs. wifi data does not exist.
Oh yes - our usage meter is available for OSX, mobile app works on iOS devices for tracking HughesNet usage. I assumed and thought we were talking about the native usage meter (or lack of) because Windows desktops (8 and up? 10 only?) have the metered connection option as well as data tracker per app. But now we know both sides