Indeed I was referring more to Android. When I was leading a dev team I studied closely what was running in the system processes when multiple apps are opened. Because of the way programmers want their apps to be prioritized on top of others, there was a lot of background processes choking up the ram. But that was a few yrs ago. I believe the newer Android OS has incorporated a feature to force inactive apps to shut down after 30 mins or so..
Understand about the push notifications part. Many readers have pointed it out, some very angrily! LOL. But again when I was studying Android 3 yrs ago I noticed the apps ran background processes constantly, especially for social apps or utility apps with feeds. I’m not sure if that qualifies as push notifications technically but it’s what I’ve noticed.
iOS doesn’t have this problem I believe. Well designed from the get go..