The .apk+ extension that you are seeing in some Android app backups is a file extension used by the Helium Backup app. Helium Backup is a popular backup tool for Android devices that allows users to backup and restore their apps and data without requiring root access.
The .apk+ extension is used by Helium Backup to indicate that the APK file has been backed up along with its associated data. The plus sign (+) is used to differentiate these backup files from regular APK files.
When you use Helium Backup to create a backup of an app, it creates a backup file with the .apk+ extension that contains both the APK file and the app’s data. This allows you to restore the app along with its data, preserving your app settings, user data, and other app-specific information.
It’s worth noting that Helium Backup is not the only app that uses this file extension. Some other backup and restore apps may also use the .apk+ extension to indicate that a file contains both the APK and data for an app. However, the use of this file extension is not standardized across all backup tools, so it’s important to check the documentation of the backup tool you are using to understand how it stores and restores app data.
So you can rename apk+ to zip, extract base.apk from the zip and install as normal.