How to do I change all my default save directory to one folder? Is there an app or settings (asus zenfone zoom) to change all my default save directory to one folder. This folder will be the "inbox" of my phone.
The short answer: No.

The long answer: Yes, but you must root your device, and it depend of wich version of Android is installed on your phone, that cancel your warranty.
If your device is rooted or if you accept to root it, you can install Xposed (that make fail your SafetyNet), and install this module: https://forum.xda-developers.com/xposed/modules/mod-downloads2sd-t2656338.

This possibility may ultimately prove non-functional if you are using a recent version of Android.

Apart from this possibility, there are no other options.
If you find another one, don't hesitate to share it here.