I have also tried in_app_update 1.1.11 and upgrader 2.4.0.
For upgrader:
I also don't know how to use Appcast, so I use the iTune api to get my app version in the App Store then pop up the install dialog. However, the iTune api is so slow to update. Therefore I just use Firebase remote config to set the version once the app is published.
For in_app_update:
I also don't know how to use it. I have add the Play Core in app/build.gradle
implementation 'com.google.android.play:core:1.8.0'
Nothing pop up even I downloaded the old version from the Play Store and then publish a new version.
Update:
I found that in_app_update actually work just by following its example code. I don't have any update pop up before because my Android device is Mi 9, seems there is some bug in it. The update pop up can be seen in the release version of other brand of devices
Since I really want to use the Play Store in-app-update, so for android I did use the remote config... waiting for someone to have the guides