adb shell pm list packages
adb shell pm path com.google.android.youtube
adb pull /data/app/com.google.android.youtube-1/base.apk .
If it says “xxx does not exist”, then you can try copy the file to sdcard
adb shell HWEVA:/ $ cp /data/app/com.google.android.youtube-1/base.apk /sdcard/
Then pull out the APK file
adb pull /sdcard/base.apk .