I have an app installed on my android phone that does not exist in the play store any more. Is there a way to create a backup, so I could install it on other devices?
You can get it with adb on non-rooted android: https://gist.github.com/ctrl-freak/24ac0e61b7cf550a6945
adb explorer adds a pretty intuitive (but slightly janky) interface.
Basically any of the old app back programs on phones just packs them back into APKs and stores them
Just search for something like APK backup or app backup on the Google Play store and you will get hundreds of working results usually the hardest part is just finding whatever bullshit folder they decided to shove them in
This just works fine.
https://play.google.com/store/apps/details?id=mobi.infolife.appbackup&hl=en&gl=US
Lucky patcher has an option to backup apks https://www.luckypatchers.com/download/