It’s very unlikely the OS actually kills apps in the background as that would legitimately break many apps and is a source of frustration from other OEMs.

There’s a difference between killing an app and putting it into a less active state.

When you swipe an app away from your recent lists, it’s not actually killing it, its just putting it in a different state.

When your force stop an app from its info under settings, you’re actually killing it. Nothing about it is alive.

When you actually kill an app, things like alarms stop functioning. The app needs to be alive for the alarm to function. Even so much that when you set an alarm on your phone, you need to set the alarm again after rebooting as they arent permanently stored and if the phone is rebooted the app needs to be woken up and the alarms re set. There’s a whole development workflow to do that.

There was a brief period many years ago when an OEM actually force killed an app when swiped away from recents without fully understanding the implications and they later reverted the change.

Push notifications of any type would also completely cease functioning.