I'm trying to figure out a way to track app usage on my phone without downloading an app that is going to share my personal data. At the bottom of Battery settings, there is a list called "App usage since full charge" which lists about a half dozen apps, each with a % to the right (I guess % battery used), and a "used for X minutes" line underneath. What does the X refer to exactly? Is that the time I'm actively using the app, or the amount of time when the app was using system resources (which I assume could include running in the background).
Percentage of battery used - if you had charged fully and check when the battery is indicating 70 %, i.e., a 30 % drop and it says app xyz used 10% it means that xyz had used 10% of 30% battery expended
Coming to your main question - it means the total power used by the battery whether in foreground or background , not the time you have used it. For instance, you can check your mail and when you go to another app, mail app goes to background and may continue to sync using battery. Same with chat or any other app which uses sync, scheduled background activities, Wakelocks or other processes. In Android 8.0 (Oreo) , if you tap the app on the battery usage section you will see clearly the actual usage while in active use and while in background as shown