How exactly do Firebase Analytics handle session d

I'm implementing some basic usage statistics for the first time in an Android/iOS app using Firebase Analytics, but the docs seem a little ambiguous to me on how exactly session durations are calculated and how sessions are timed out, and the 24-hour cycle for seeing results does not help in understanding by trial-and-error.

I'm only interested in a very simple statistic for now - the time users spend in the app. What exactly happens when the app is "minimised"/"in the background" - does that count as active usage or the same as the app being closed - is not so important right now, but if you have something to add in this regard in the context of the question, please do.

From what I understand such an easy statistic should probably be available out-of-the-box with Firebase Analytics, so I'm currently not using any custom events or anything fancier than simply linking Firebase Analytics into the projects, adding the configuration files, and calling FirebaseAnalytics.getInstance(this); on Android and [FIRApp configure]; on iOS.

I'm seeing first_open, session_start and app_remove events show up in the console as expected, some aggregate session data in the dashboard, etc, and I also know there is a configurable session timeout available.

But what is unclear to me is, when exactly does a user session expire - does Firebase automatically keep it open as long as the app is open, or do I need to make sure to post some artificial "keep-alive" events to prevent user sessions from expiring while a user is still actually using the app.

Assuming the following:

I send no explicit custom events using Firebase, as this is not explicitly required by the app.
I only touch Firebase once during a single run of the app - initialising it on startup.
The user stays in the app for a long time (let's say 2 hours - much longer than the default 30 minute session timeout), without interacting with it - e.g. reading something, watching a video, etc, none of this generates any events that Firebase can see, but the app prevents the screen from locking, the app remains in foreground.

Will that count as a single 2 hour session for the user? Will it only count as a 30 minute session since the session expires after 30 minutes and no events are generated to keep it alive? Will something else happen?

Bonus question: what happens if the user exits the app for a time shorter/longer than the session timeout?

Bonus bonus question: does something change if instead of exiting from the app, it is simply put into background?

Thanks!

Quick answer: The scenario you describe will result in a single 2-hour session.

Sessions are based on the time that an app's activity is the current activity. There is no need to send events; the period from the time an activity resumes to the time it is suspended is counted as engagement time. At the end of an hour of continuous engagement an engagement event will be logged but this does not end the session. A session expires when there is a continuous period (30 minutes by default) that none of the app's activities is the current activity. If the user exits the app for a shorter period than the session timeout and then restarts it, the session continues. If the user exits the app for longer than the session timeout, then the session ends. No change between exiting the app and putting it in the background. If it's not the thing the user is looking at, it doesn't count as engagement for the app to be running.