After a comment regarding Home Theater v4.0.39 is not working on X8 I’ve rechecked my creation process. By mistake I’ve created the modded APKs with API Level 25 (Android 7.1 Nougat) and not with API Level 23 (Android 6 Marshmallow).*
Modded APK
Zidoo Music Player v6.9.22 – McBluna.net
With the 
beta firmware v6.0.90(_G) Zidoo updated Music Player to version v6.9.22.
I’ve created a modded APK for the Android 6/7.1 devices.
Zidoo Home Theater v4.0.39 – McBluna.net
With the beta firmware v6.0.90(_G) Zidoo updated Home Theater to version v4.0.39.
I’ve created a modded APK for the Android 6/7.1 devices.
Zidoo Home Theater v4.0.21 – McBluna.net
With the Z1000 Pro beta firmware v6.0.45_G Zidoo updated Home Theater to version v4.0.21
I’ve created a modded APK for the Android 6/7.1 devices based on the Android 9 code from the Z1000 Pro beta firmware v6.0.45_G
Zidoo Home Theater v4.0.20 – McBluna.net
With the beta firmware v6.0.45_G Zidoo updated Home Theater to version v4.0.20
I’ve created a modded APK for the Android 6/7.1 devices based on the Android 9 code from the beta firmware v6.0.45_G
Zidoo Music Player v6.7.6 – McBluna.net
With the Z9X/Z10 Pro Beta Firmware v6.0.45 Zidoo updated Music Player to Version v6.7.6
I’ve created a modded APK for the Android 6/7.1 devices based on the Android 9 code from the Z9X firmware v6.0.45_G beta
Modded APKs for Android 6/7.1 devices based on Android 9 firmware
It took me three days to figure out how to create working APKs for the Android 6/7.1 devices based on the Android 9 code from the Z9X firmware v6.0.35.
Update 2020-09-10: Since Zidoo released an update for Z9S with even more current versions of their apps, the use of modded APKs is not necessary for now.