Replicant

1. identify your app by going in app panel, then maintaining a finger on it until "Information" menu appears, and then slide it to this menu, the name will be displayed
2. from your computer where is installed adb, run

adb shell pm path org.fossify.contacts
package:/data/app/org.fossify.contacts-1/base.apk

The path will be the full path where is the APK file (prefixed with "package:").

3. on your computer, rune

adb pull <APP_PATH> <LOCAL_PATH>

where APP_PATH is the path displayed in 2 and <LOCAL_PATH is the directory where you want to download the APK file

source: https://stackoverflow.com/questions/43342330/get-apk-of-installed-app-with-adb-command