why would swiping away an app not kill it? why would you do that? leave it be until it's done wtf
how much RAM you got? on my two A15 phones with 6 and 8 GB RAM nothing extraordinary happens in that regard, whereas my A15 4 GB RAM tablet can't handle a lot of open apps and OOM kills some in the background.
edit: you seem to be trying to run android apps like desktop apps. that doesn't work here. how things work is most apps are dormant when they don't have focus and when they receive a push message e.g. "you have a new message", their handler wakes up, fetches stuff from the server and updates local state (and then optionally displays the notification).
so you either need to install unified push and get apps that support it or have e.g. microG implement a subset of Play services so that GCM/FCM works with "normal" apps. the third option, what you may be doing, is having every app excluded from sleep and doing their own updates checking.
the example where you close an app and then go "why app closed" is unrelated.